Android app silently stops sending reading position to the server (no errors in the logs)

On a Boox e-ink device, reading position stopped syncing to my self-hosted server. It started with a genuine connectivity problem — the VPN client was being killed in the background, so the app logged that it couldn't resolve my server's hostname. After fixing that, the app can reach the server again and pulls down newly added books fine, but position updates stop entirely: no further sync attempts appear in the logs at all, in either direction.

Resolution: Two separate things bite here. First, aggressive battery/background management on Android (Boox devices are notorious for this) will kill your VPN client, which produces real DNS/connection errors in the app log — allow background processes for the VPN app to stop that. Second, once the sync listener has failed it can get stuck in a broken state and never retry, which is why you see complete silence in the logs rather than errors, while other server calls still work. Force quit the app (long press the icon → App info → Force stop) and reopen it; if that alone doesn't restore it, reboot the device. That clears the stuck listener and position sync resumes in both directions. You should not have to do this — it's a bug in the sync listener's recovery, not intended behavior — but it is the reliable workaround.