Login prompt after every reboot

Nextcloud version (eg, 20.0.5): Client 3.4.3 Snap
Operating system and version (eg, Ubuntu 20.04): Ubuntu 16.04.7

After every system reboot the nextcloud client asks me to grant access again. Nextcloud-app opens a window:

You have been logged out by xxx@cloud.xxx.de as user xxxxx. Please login again.

Followed by a shortcut opening the default browser firefox and https://cloud.xxx.x/index.php/login/v2/flow

 Connect to your account
 Please log in before granting client access to your Nextcloud account.
 
If you are not trying to set up a new device or app, someone is trying to trick you into granting them access to your data. In this case do not proceed and instead contact your system administrator.

After clicking the login button I can grant access again.

Also another tab is opened by default:

Internal Server Error

The server was unable to complete your request.

If this happens again, please send the technical details below to the server administrator.

More details can be found in the server log.
Technical details

    Remote Address: 
    Request ID: 6GArzb4A16n5vgGSG1Y2

Part of client log:

2022-02-28 14:13:16:519 [ info nextcloud.sync.networkjob /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/abstractnetworkjob.cpp:361 ]:	OCC::JsonApiJob created for "https://cloud.xxx" + "ocs/v2.php/apps/activity/api/v2/activity" "OCC::ActivityListModel"
2022-02-28 14:13:16:519 [ debug nextcloud.sync.networkjob /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/abstractnetworkjob.cpp:298 ]	[ OCC::AbstractNetworkJob::slotFinished ]:	Network job OCC::JsonApiJob finished for "ocs/v2.php/apps/activity/api/v2/activity"
2022-02-28 14:13:16:588 [ info nextcloud.sync.credentials.webflow /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/gui/creds/webflowcredentials.cpp:425 ]:	request finished
2022-02-28 14:13:16:588 [ info nextcloud.sync.networkjob.jsonapi /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/networkjobs.cpp:881 ]:	JsonApiJob of QUrl("https://cloud.xxx/ocs/v2.php/apps/activity/api/v2/activity?since=359658&limit=50&format=json") FINISHED WITH STATUS "OK"
2022-02-28 14:13:16:590 [ debug nextcloud.sync.networkjob /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/abstractnetworkjob.cpp:298 ]	[ OCC::AbstractNetworkJob::slotFinished ]:	Network job OCC::JsonApiJob finished for "ocs/v2.php/apps/activity/api/v2/activity"
2022-02-28 14:13:21:599 [ debug nextcloud.gui.account.state /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/gui/accountstate.cpp:262 ]	[ OCC::AccountState::checkConnectivity ]:	"xxx@cloud.xxx" The last ETag check succeeded within the last  30 s ( 6 s). No connection check needed!
2022-02-28 14:13:25:283 [ debug default unknown:0 ]	[ unknown ]:	static bool LibSecretKeyring::findPassword(const QString&, const QString&, QKeychain::JobPrivate*)
2022-02-28 14:13:25:297 [ info nextcloud.sync.credentials.keychainchunk /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/creds/keychainchunk.cpp:346 ]:	Backend unavailable (yet?) Retrying in a few seconds. "An AppArmor policy prevents this sender from sending this message to this recipient; type=\"method_call\", sender=\":1.71\" (uid=1000 pid=1672 comm=\"/snap/nextcloud-desktop-client/33/usr/bin/nextclou\") interface=\"org.freedesktop.Secret.Service\" member=\"OpenSession\" error name=\"(unset)\" requested_reply=\"0\" destination=\":1.1\" (uid=1000 pid=1118 comm=\"gnome-keyring-daemon --start --components pkcs11,s\")"
2022-02-28 14:13:35:599 [ debug default unknown:0 ]	[ unknown ]:	static bool LibSecretKeyring::findPassword(const QString&, const QString&, QKeychain::JobPrivate*)
2022-02-28 14:13:35:603 [ warning nextcloud.sync.credentials.keychainchunk /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/creds/keychainchunk.cpp:360 ]:	Unable to read "proxy-password" chunk "0" "An AppArmor policy prevents this sender from sending this message to this recipient; type=\"method_call\", sender=\":1.71\" (uid=1000 pid=1672 comm=\"/snap/nextcloud-desktop-client/33/usr/bin/nextclou\") interface=\"org.freedesktop.Secret.Service\" member=\"OpenSession\" error name=\"(unset)\" requested_reply=\"0\" destination=\":1.1\" (uid=1000 pid=1118 comm=\"gnome-keyring-daemon --start --components pkcs11,s\")"
2022-02-28 14:13:35:604 [ warning nextcloud.sync.credentials.keychainchunk /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/libsync/creds/keychainchunk.cpp:298 ]:	ReadPasswordJob failed with "An AppArmor policy prevents this sender from sending this message to this recipient; type=\"method_call\", sender=\":1.71\" (uid=1000 pid=1672 comm=\"/snap/nextcloud-desktop-client/33/usr/bin/nextclou\") interface=\"org.freedesktop.Secret.Service\" member=\"OpenSession\" error name=\"(unset)\" requested_reply=\"0\" destination=\":1.1\" (uid=1000 pid=1118 comm=\"gnome-keyring-daemon --start --components pkcs11,s\")"
2022-02-28 14:13:35:611 [ debug nextcloud.sync.database.sql /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/common/ownsql.h:145 ]	[ OCC::SqlQuery::bindValue ]:	SQL bind 1 3
2022-02-28 14:13:35:611 [ debug nextcloud.sync.database.sql /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/common/ownsql.cpp:295 ]	[ OCC::SqlQuery::exec ]:	SQL exec "SELECT path FROM selectivesync WHERE type=?1"
2022-02-28 14:13:35:612 [ debug nextcloud.sync.database.sql /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/common/ownsql.h:145 ]	[ OCC::SqlQuery::bindValue ]:	SQL bind 1 3
2022-02-28 14:13:35:612 [ debug nextcloud.sync.database.sql /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/common/ownsql.cpp:295 ]	[ OCC::SqlQuery::exec ]:	SQL exec "SELECT path FROM selectivesync WHERE type=?1"
2022-02-28 14:13:35:612 [ info nextcloud.gui.account.settings /build/nextcloud-desktop-client/parts/nextcloud-desktop-client/src/src/gui/accountsettings.cpp:1197 ]:	Account "xxx@cloud.xxx" Client Side Encryption false
2022-02-28 14:13:35:614 [ warning default unknown:0 ]:	QFSFileEngine::open: No file name specified
2022-02-28 14:13:35:614 [ warning qt.svg unknown:0 ]:	Cannot open file '', because: No file name specified

Nevermind. One should read packages notes.

Following are needed interfaces for the client. When these interfaces are not connected, the client won't work as expected:

   sudo snap connect nextcloud-desktop-client:mount-observe 
   sudo snap connect nextcloud-desktop-client:network-manager-observe 
   sudo snap connect nextcloud-desktop-client:password-manager-service

In additon, if you get the keyring prompt, you might want to consider this: