@TMC according to this post, a user managed to fix a very similar sync issue by deleting an encryption key, as they somehow had more than one. If you find there is more than one encryption key and you want to attempt this, please make a jex backup of all your notes first, in case something goes wrong and you can no longer decrypt your notes
The sharepoint error could just be a red herring, as you mentioned it goes away when you try to sync again