No Backup plugin is working

Every backup/clone plugin I tried is not working. The backup is always 0 byte in size. I tried WP “WP Clone”, “Duplicator” and “UpDraft”. Same problem with every plugin.
In the past I always used Duplicator and it worked flawlessly. In fact I have a backup of the site for my local development and Duplicator works there.
So it has to be something with the Server. What kind of setting could be the problem here?
I think the problems started after configuring “Super Cache” and “iThemes Security” but neither deactivating/deleting these plugings nor setting the configuration to default settings helped.