The "SQLite_Busy: database is locked" error is not possible within the main app. Only way it can happen is if something external is accessing the database (either another instance of Joplin or a really poorly written plugin, but I don't think such plugin exists). It might just be that you need to restart your computer to clear any bad process.
Also the reboot did not work. I do sync with onedirve. I did saw theire, that this seems to be an issue with it. Yesterday evening and last night it worked well. Then I did several reboots, cleaned the chache, rebuild the sync as desribed.... But still got the same error.
First of all, you should disable all plugins to narrow down the problem and determine whether it is a bug in the plugin or a bug in joplin itself. If it is a plug-in problem, a dichotomy may be needed to determine which plug-in.