This is an old post, but while running the Windows client 3.13.0, I ran into this same error. I thought I’d share the fix I found in case anyone else comes across this post with this issue.
My setup is simplified. This is a local folder on a non-root local drive, but trying to sync it to my Nextcloud server. I kept getting this same error AFTER the initial sync worked.
I hadn’t changed a thing, except a recent upgrade to the client.
So, after some testing and comparing the permissions with the user’s folder on this drive with those of another folder that did NOT get this same message, it turned out that the upgrade appeared to have randomly inserted a new permissions limitation in the security tab. It literally set DENY to all local users. As soon as I removed this new entry, everything worked swimmingly.
Hope this helps someone else in the future.
Cheers!