Nextcloud sync failing, “Connection Closed/Operation Canceled” Ubuntu

I’ve got Nextcloud snap server version 15.0.11 on my Ubuntu Server 18.04.3 and a Nextcloud Client on Windows 10 and a client on Ubuntu 18.04.
The Client on Windows works fine, but the client on Ubuntu doesn’t work correct.
When I installed it and configured it, it won’t sync and says “connection closed”.

How can I fix this problem.