The client with a problem emits an Unknown error: (400) while attempting to upload resources that haven't been modified.
I'm not asking for help to fix the problem, but rather assistance debugging the problem.
The first thing I tried to do was find documentation on the SQLite schema housed in this file.
$HOME/.config/joplin-desktop/database.sqlite
Is there any documentation on this schema? Is there documentation on the Joplin server schema as well? Documentation on the android app's schema would also be helpful, as I'm dealing with issues on that app.
If necessary, I can infer the purpose of various fields from the code and start some documentation if it doesn't exist. I won't go blindly into debugging without understanding the true meaning of all the various schemas.
Any knowledge beyond this documentation is welcome.
I can't tell from the debug log, but maybe you'll have more luck by checking the server log. Does your server allow resources of that size to be uploaded? (it's not particularly large at 102592 bytes but still worth checking)
The reason I'm asking for the error message is that I think you're not looking in the right place. DB schema, how to build the app, etc. won't help you to figure out that error which is most likely a problem with the server.