Sync with Android App doesn't download all files


I’m trying to sync a bigger directory structure with audio files (~8 GB in ~1400 files) to an older Android phone (Huawei with Android 4.0.4). I’m using Nextcloud 11 (based on UCS).

The sync downloads about 30% of the files and reports failures for the others in the notification bar. I tried to identify a few files that don’t work and can reproduce the behavior for smaller subsets. One example: there is one directory (one music album) containing 13 mp3 files and a jpg out of which 3 mp3 files are actually synced to the phone. All others fail, even if I select only one of them and try a “download”.

I can download these files using the web client.

Are there any known limitations? How can I debug this?