UPDATE:
ok, so the issue for me appears to be the bruteforce protection feature on the NC server. I don’t know all of the options, but if you can use phpmyadmin to access your database tables and clear out the entries inside the “oc_bruteforce_attempts” table, it should solve the issue.
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I’m seeing this issue suddenly also. I cannot explain what is happening. I made NO CHANGES to the client.
My other clients are JUST FINE.
I have been doing work on my server, but the server application is isolated from the OS packages (unless it is fundamental). And I did not change the NC server.
Here is the kicker. If you add a new account in the NC client - it will sync without issue. If you close and re-open the client, it will fail to sync. Repeat the add of that same user and it will sync again.