Sync issue from notes app to nextcloud server: `UnknownErrorException: failed to connect to`

I cannot sync my notes from my phone to my nextcloud server (version 26.0.1). I attached the error log from notes.
I am unsure for how long I see the issue, at least some days probably some weeks.

App Version: 4.0.0
App Version Code: 40000090
App Flavor: play

Files App Version Code: 30240290

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OS Version: 4.14.186+(R.GDPR.f1ba30-5fd8)
OS API Level: 31
Device: OP515BL1
Manufacturer: OnePlus
Model (and Product): DN2103 (DN2103EEA)

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java.lang.RuntimeException: com.nextcloud.android.sso.exceptions.UnknownErrorException: failed to connect to <I REMOVED MY DOMAIN AND IP ADDRESS> (port 443) from /<I REMOVED THE IP ADDRESS> (port 44539) after 60000ms
	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$RunnableAdapter.call(Executors.java:463)
	at java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1137)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:637)
	at java.lang.Thread.run(Thread.java:1012)
Caused by: com.nextcloud.android.sso.exceptions.UnknownErrorException: failed to connect to  <I REMOVED MY DOMAIN AND IP ADDRESS> (port 443) from /<I REMOVED THE IP ADDRESS> (port 44539) after 60000ms
	at com.nextcloud.android.sso.api.AidlNetworkRequest.performNetworkRequestV2(AidlNetworkRequest.java:188)
	at com.nextcloud.android.sso.api.NextcloudAPI.performNetworkRequestV2(NextcloudAPI.java:199)
	at com.nextcloud.android.sso.api.NextcloudAPI.lambda$performRequestObservableV2$1$com-nextcloud-android-sso-api-NextcloudAPI(NextcloudAPI.java:129)
	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

Start with NotesServerSyncTask.java at pullRemoteChanges (line 219) and run (line 96), then trace the Nextcloud SSO network calls shown in the stack trace. Reproduce the timeout if possible and determine why syncing cannot connect to the server; done requires a verified explanation and confirmation that sync succeeds.