I started getting these errors as well. They appear quite often and require me to re-create notes locally, or edit files manually on the server. It seems to have started since updating to Nextcloud 19, not sure if this is a coincidence. It happens so often that I have to fix files 5-10 times a day.
I use Redis for file locking. Doesn’t joplin have its own file locking system as well?
I get other similar issues as well, like “Invalid property is_shared”