android notes app can't sync `NextcloudApiNotRespondingException`

Environment:

• Ubuntu 22 LTS upgraded to the latest possible version
• apache2
• php 8.1
• mariadb
• Nextcloud Hub 5 (27.0.2)
• Notes app: v4.8.1
• Android 12
• notes app for android: latest version just installed from google play (v4.0.0 March 21st, 2023)
• battery optimization is off

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

Files App Version Code: 30250090

---

OS Version: 4.14.186(1685598849)
OS API Level: 31
Device: Power_Armor14_Pro
Manufacturer: Ulefone
Model (and Product): Power Armor14 Pro (Power_Armor14_Pro)

---

java.lang.RuntimeException: com.nextcloud.android.sso.exceptions.NextcloudApiNotRespondingException: Nextcloud Files app API is not responding. This is most likely due to battery optimizations on your phone. Try to disable the battery optimization for both apps (Nextcloud Android and this app). In all other cases please try to clear the storage of both apps.
    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.NextcloudApiNotRespondingException: Nextcloud Files app API is not responding. This is most likely due to battery optimizations on your phone. Try to disable the battery optimization for both apps (Nextcloud Android and this app). In all other cases please try to clear the storage of both apps.
    at com.nextcloud.android.sso.api.AidlNetworkRequest.waitForApi(AidlNetworkRequest.java:162)
    at com.nextcloud.android.sso.api.AidlNetworkRequest.performAidlNetworkRequestV2(AidlNetworkRequest.java:298)
    at com.nextcloud.android.sso.api.AidlNetworkRequest.performNetworkRequestV2(AidlNetworkRequest.java:180)
    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 the stack trace in NotesServerSyncTask.java, especially pullRemoteChanges at line 219, and review how the app calls the Nextcloud Files app API during sync. Reproduce the Android 12 sync failure with the listed environment and determine whether the API responds; done means syncing completes without NextcloudApiNotRespondingException.