Note: Errorcode 20 is a very generic code which can indicate very different situations.
When trying to change the password via the web interface (UMC) some or all users are constantly getting an errorcode 20 message.
This issue might be related to a bug. To fix it temporarily restart UMC:
systemctl restart univention-management-console-server
Once verified and the issue is not gone, consider to check these articles as well: