Three things I'd try:
- Mobile client > Configuration > Tools > Sync Status > If there are errors, press on "Retry all" (unlikely, see n.2).
- If your desktop works correctly, have you tried re-uploading your notes? For that, make sure your desktop is fully synced (and I'd backup your notes if you haven't already automatically). Then go to Tools > Options > Synchronization > Show Advanced Settings > Re-upload local data to sync target.
- If you want to try something else, you could try using an older build of Joplin on F-Droid and see if it works there.
P.s. I think you should input an encryption master password only after having done a full sync on the device and then entering it in the orange prompt that appears. So your problem might stem from a badly configured E2EE. In that case, if the things above don't work, I'd suggest turning off E2EE, synching all your notes to your devices and then turning it back on. Hang in there!
Edit: Three things :)