Hello @luckow
For context and documentation, is this a Cloudron app or a custom app?
In both cases, can you please attempt the following.
Put the app into recovery mode
Attempt to create an app backup
If this works, I suspect the app is creating/deleting/moving files so fast that while a snapshot is created the file is already changed.
Under normal circumstances this should not be a problem, so if this works, we will have to look into it.
I also assume this issue is specific to the rsync type backup, since the tgz style will not care about such a scenario.