So, my android client has not been able to decrypt for some time (syncing from Dropbox -- my desktop Joplin instance is fine). This happened after I changed phones. I blew everything away on mobile and tried again and then again and then again ... It resyncs everything under the covers, but doesn't decrypt all of it (missing key somewhere?) and thus all I have are the default sample documents. Any errors are silent at the normal user level. The log file are ... cryptic.
I'm thinking I need to nuke the Dropbox database, re-encrypt, and resync across the desktop and mobile. All I need to do is save the .jex file and reimport from that, right? I'd lose history, but ... whatever. Essentially a disaster recovery from backup. I would re-encrypted with a new master and get the desktop right and then try again with the android app afterward.
You could first try Re-upload local data to sync target under Options > Synchronisation > Advanced Settings. If that still doesn't work, I would indeed do File > Export all > JEX - Joplin Export File, wipe everything (locally and in the dropbox), and then re-import the JEX File.
Proceed with caution, be sure the JEX is complete and not damaged, otherwise you will loose data.
I did your re-upload local data suggestion. Then blew away the mobile app, re-installed, and tried again and that seemed to fix it. Now, I don't know for sure if I didn't do something else differently, but ... I am now back in a working state.
I also moved the Joplin and joplin-desktop folders into a backup directory and tested my last saved JEX. It appeared like a complete backup.
I may just move to Joplin-Cloud. I already contribute $5 per month to Laurent via github donations. I may as well subscribe to the cloud service.