Backup of virtual server failed because of DAV

When backing up all virtual servers one of them fails with the following error:

Backing up DAV users file .. .. no users file found!

I have looked at the Services -> DAV authentication and the server that fails has 0 users with DAV access. All other virtual servers have 1 user with DAV access so that might be the problem.

How can I add a user with DAV access to the virtual server?

What the hell, I have disabled the DAV, nobody uses it anyways. Now the backup went smooth.

Same problem here…
Is there a way to create the DAV user File?

Backing up DAV users file ..
.. no users file found!

Otherwise i get an error every night…