Mounting Image from Webserver fails (sort of)

When trying to mount an image via the webinterface either the server crashes or it says something like “Could not mount image see log for more information”.

The log does not contain helpful information. Interestingly the mounting process via guestmount does work as intended. The image is mountet to a mnt0 location is the same dir as the backups and can be accessed via console.

They can be mounted/unmounted via terminal without a problem.

System:
Ubuntu 18.04
ZFS filesystem
Server does run as root

Hope somebody can help me with this.

PUSH

This does not seem to be covered before.
I only found one entry which discussed a problem with supermin which is not the case here.

libguestfs-test-tool does not find any proplem (in debug mode)
urbackup_mount_helper does mount images aswell without an error.

Regards