Another potential solution here:
"In the Nextcloud External Storage setting, you have to enable "show hidden files".
Then sync each Joplin instance.
After syncing each instance, you can disable "show hidden files" and it still syncs fine."
And also:
"I just had the “403 forbidden” mkcol today and this helped me a lot! I logged on the nextcloud directory, under nextcloud>user>files>desired directory and changed the owner of the directory to “www-data” by using "chown -R www-data:www-data /path/to/desired/directory"
Also, I notice in your OP that you mentioned you created a new user which you synced another profile with. What happens if you use the same user for which you originally created the profile for which you are trying to sync?