Failure to sync Notes on Android

Hello
On my Redmi Note Pro 9, the android Notes app fails to synchronize since last week. The server is a nextcloud free plan at cloud.woelkli.com. Everything else seems to work fine (eg. files sync on Android, web app, nextcloud app on Linux), only Notes sync fails, only on Android.
I tried deleting all the data of the Notes app on Android, relinking it to the server successfully downloads the Notes but next sync will fail again.
Not sure it’s related but it started when I added my first tags/categories to my notes (from the web app). I tried removing the tags (from the web app), it doesn’t help.

The sync failure log is below:

App Version: 4.2.1
App Version Code: 40020190
App Flavor: fdroid

Files App Version Code: 30290090 (PROD)OS Version: 4.14.117-perf-g734af9b(V12.0.4.0.QJZEUXM)
OS API Level: 29
Device: joyeuse
Manufacturer: Xiaomi
Model (and Product): Redmi Note 9 Pro (joyeuse_eea)java.lang.RuntimeException: com.nextcloud.android.sso.exceptions.NextcloudHttpRequestFailedException: La requête HTTP a échoué avec le statut HTTP :502
at io.reactivex.internal.util.ExceptionHelper.wrapOrThrow(ExceptionHelper.java:46)
at io.reactivex.internal.observers.BlockingMultiObserver.blockingGet(BlockingMultiObserver.java:93)
at io.reactivex.Maybe.blockingGet(Maybe.java:2321)
at io.reactivex.Observable.blockingSingle(Observable.java:5381)
at it.niedermann.owncloud.notes.persistence.NotesServerSyncTask.pullRemoteChanges(NotesServerSyncTask.java:219)
at it.niedermann.owncloud.notes.persistence.NotesServerSyncTask.run(NotesServerSyncTask.java:96)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:462)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
at java.lang.Thread.run(Thread.java:919)
Caused by: com.nextcloud.android.sso.exceptions.NextcloudHttpRequestFailedException: La requête HTTP a échoué avec le statut HTTP :502
at com.nextcloud.android.sso.api.AidlNetworkRequest.performNetworkRequestV2(AidlNetworkRequest.java:189)
at com.nextcloud.android.sso.api.NextcloudAPI.performNetworkRequestV2(NextcloudAPI.java:165)
at com.nextcloud.android.sso.api.NextcloudAPI.lambda$performRequestObservableV2$0(NextcloudAPI.java:105)
at com.nextcloud.android.sso.api.NextcloudAPI.$r8$lambda$2CK7Y9lXWWrtN8jgT_iNTMeTrHU(Unknown Source:0)
at com.nextcloud.android.sso.api.NextcloudAPI$$ExternalSyntheticLambda0.subscribe(Unknown Source:6)
at io.reactivex.internal.operators.observable.ObservableFromPublisher.subscribeActual(ObservableFromPublisher.java:31)
at io.reactivex.Observable.subscribe(Observable.java:12284)
at io.reactivex.internal.operators.observable.ObservableSingleMaybe.subscribeActual(ObservableSingleMaybe.java:31)
at io.reactivex.Maybe.subscribe(Maybe.java:4290)
at io.reactivex.Maybe.blockingGet(Maybe.java:2320)
... 8 more
Caused by: java.lang.IllegalStateException: <!DOCTYPE html>
<html dir='ltr' lang='en'>
  <head>
    <meta http-equiv='Content-Type' content='text/html;charset=utf-8'/>
    <title>The website is under maintenance</title>
    <style>
      body {margin:0;font-family:arial,sans-serif;color:#8c8c8c;line-height:1.5em;text-align:left;}
      body, html {height:100%;}
      #top {position:absolute;top:0;left:0;width:100%;padding:0.5em 0;background-color:#111;box-shadow:#424242 1px 0 5px;}
      #inside {color:#f3f3f3;font-weight:bold;padding-right:1em;padding-left:1em;}
      #inside a {text-decoration:none;color:#f3f3f3;}
      #logo {float:left;display:inline-block;}
      #logo a, #logo img {vertical-align:middle;}
      #logo img {margin-right:0.5em;}
      #pnf {clear:both;display:block;overflow:auto;width:100%;min-height:100%;margin-bottom:-6em;background:url('https://woelkli.com/static-images/cow.jpg') 0 0/cover no-repeat transparent;}
      #pnf:after {display:block;height:8em;content:'';}
      #dialogue-outer {float:left;margin:8em 2em 7em 3em;padding:2em;width:40%;background-color:rgba(250, 250, 250, 0.5);-moz-border-radius:0.4em;-webkit-border-radius:0.4em;-ms-border-radius:0.4em;-o-border-radius:0.4em;border-radius: 0.4em;}
      #dialogue {padding:2em;background-color:#fafafa;-moz-border-radius:0.4em;-webkit-border-radius:0.4em;-ms-border-radius:0.4em;-o-border-radius:0.4em;border-radius:0.4em;-moz-box-shadow:0 2px 2px rgba(0, 0, 0, 0.196),0 1px 5px rgba(0, 0, 0, 0.196);-webkit-box-shadow:0 2px 2px rgba(0, 0, 0, 0.196),0 1px 5px rgba(0, 0, 0, 0.196);-ms-box-shadow:0 2px 2px rgba(0, 0, 0, 0.196),0 1px 5px rgba(0, 0, 0, 0.196);-o-box-shadow:0 2px 2px rgba(0, 0, 0, 0.196),0 1px 5px rgba(0, 0, 0, 0.196);box-shadow:0 2px 2px rgba(0, 0, 0, 0.196),0 1px 5px rgba(0, 0, 0, 0.196);}
      h1 {font-size:1.5em;font-weight:bold;text-shadow:rgba(255, 255, 255, 0.8) -1px -1px 2px, rgba(163, 163, 163, 0.8) 1px 1px 1px;}
      h2 {margin-top:1.5em;font-style:italic;}
      a {color: #275CA1;text-decoration: none;}
      #info {height:6em;margin:0 auto;background-color:#275ca1;-moz-box-shadow:#000 -1px 0 5px;-webkit-box-shadow:#000 -1px 0 5px;-ms-box-shadow:#000 -1px 0 5px;-o-box-shadow:#000 -1px 0 5px;box-shadow:#000 -1px 0 5px;}
      #info ul {margin:0 auto;padding-top:2.5em;text-align:center;}
      #info ul li {display:inline;padding-right:2em;}
      #info a {text-decoration:none;color:#f3f3f3;text-shadow:rgba(0, 57, 74, 0.804) 0 1px 1px,rgba(0, 57, 74, 0.196) 0 6px 5px,rgba(0, 57, 74, 0.102) 8px 0 8px;}
      #info a:hover {text-decoration:underline;}
      #footer #bl {display:block;padding-top:0.25em;padding-bottom:0.25em;background-color:#fff;}
      #footer #bl #cloud-software {display: inline-block;width: 33%;max-width: 33%;}
      #footer #bl #cloud-software a {text-align: left;}
      #footer #bl #cloud-software img {padding-left: 1em;width: 76px;height: 50px;}
      #footer #bl #host {display:inline-block;width: 33%;max-width: 33%;text-align: center;vertical-align: middle;}
      #footer #bl #host a {height: 39px;font-size: 1em;color: #666;}
      #footer #bl #host a:hover {text-decoration:none;}
      #footer #bl #ssl {display: inline-block;width: 33%;max-width: 33%;}
      #footer #bl #ssl a {float: right;text-align: right;}
      #footer #bl #ssl img {width: 169px;height: 39px;vertical-align: middle;}
    </style>
  </head>
  <body>
    <header id='header'>
      <div id='top'>
        <div id='inside'>
          <div id='logo'>
          <a href='/en'><img src='https://woelkli.com/static-images/woelkli.png' alt='woelkli logo'></a>
        </div>
      </div>
      </div>
    </header>
    <div id='pnf'>
      <div id='dialogue-outer'>
        <div id='dialogue'>
          <h1>The website is under maintenance</h1>
          <p>The website is under maintenance and will be back soon. Please check back again in a while.</p>
          <h2>In the meantime, you can:</h2>
          <ul>
            <li><a href='https://woelkli.com/en/signup'>Get a Cloud Storage account</a></li>
            <li><a href='https://woelkli.com/en/about'>Find out more about wölkli Cloud Storage</a></li>
            <li><a href='https://woelkli.com/en/faq'>Visit our frequently asked questions page</a></li>
            <li><a href='https://woelkli.com/en/help'>Read our how-to guides</a></li>
            <li><a href='https://pro.woelkli.com'>Sign in to your wölkli PRO account</a></li>
            <li><a href='https://cloud.woelkli.com'>Sign in to your wölkli FREE account</a></li>
          </ul>
        </div>
      </div>
    </div>
    <footer id='footer'>
      <nav id='info'>
        <ul>
          <li>
            <a href='https://woelkli.com/en/features'>Features</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/packages'>Packages</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/signup'>Sign up</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/upgrade'>Upgrade</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/donate'>Donate</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/onlyoffice'>ONLYOFFICE</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/collabora-online'>Colabora Online</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/downloads'>Downloads</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/help'>Help</a>
          </li>
          <li>
            <a href='https://woelkli.com/en/faq'>FAQ</a>
          </li>
          <li>
            <a href='https://woelklimail.com/en/' target='_blank'>Encrypted e-mail</a>
          </li>
          <li>
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          </li>
          <li>
            <a href='https://woelkli.com/en/contact-us'>Contact</a>
          </li>
        </ul>
      </nav>
      <div id='bl'>
        <span id='cloud-software'>
          <a href='https://nextcloud.com/' target='_blank' rel='nofollow'>
            <img src='https://woelkli.com/static-images/nextcloud.png' alt='Nextcloud'>
          </a>
        </span>
        <span id='host'>
          <a href='https://oriented.net/' target='_blank'>© oriented.net LLC</a>
        </span>
        <span id='ssl'>
          <a href='https://letsencrypt.org/' target='_blank' rel='nofollow'>
            <img src='https://woelkli.com/static-images/letsencrypt.png' alt='SSL Secured'>
          </a>
        </span>
      </div>
    </footer>
  </body>
</html>

at com.nextcloud.android.sso.InputStreamBinder.processRequestV2(InputStreamBinder.java:440)
at com.nextcloud.android.sso.InputStreamBinder.performNextcloudRequestAndBodyStreamV2(InputStreamBinder.java:113)
at com.nextcloud.android.sso.InputStreamBinder.performNextcloudRequestV2(InputStreamBinder.java:96)
at com.nextcloud.android.sso.aidl.IInputStreamService$Stub.onTransact(IInputStreamService.java:110)
at android.os.Binder.execTransactInternal(Binder.java:1021)
at android.os.Binder.execTransact(Binder.java:994)

Thanks

<title>The website is under maintenance</title>

Looks like something to check with our hosting provider about.

Everything else works fine, I wonder why only Notes on Android would report this.

I manage to get things back to work by removing all notes (from the Linux mount app) and adding them back. I added them one by one to see which one would break things but it didn’t occur. I even added tags again without any problem. Don’t know how to debug this further. No reply from woelkli yet about the “under maintenance” message.