Mounting image from web failed, but from server shell works well

Hello,
I was trying to mount backup image via web interface. First of all, no button for mounting image was visible. So I was searching and found that this was failing:

So I have to install the missing package:

apt-get install -y libguestfs-tools

Now the test is OK:

urbackup_mount_helper test
MOUNT TEST OK

The button for mounting backup image is now visible, but when I try it the following error appears:

UrBackup Server Log:

2021-03-19 13:55:39: ERROR: Image mounting failed: Loading FUSE kernel module...
Starting VHD background process...
Waiting for background process to become available...
Mounting...
libguestfs: error: qemu-img: /tmp/libguestfsb1mEbz/overlay1: qemu-img exited with error status 1.
To see full error messages you may need to enable debugging.
See http://libguestfs.org/guestfs-faq.1.html#debugging-libguestfs
Mounting failed.
UrBackup mount process returned non-zero return code

However, the image mounting with the shell command works like a charm:

urbackupsrv mount-vhd --file /media/data2/urbackup/ASUSUX51VZ/210301-0625_Image_C/Image_C_210301-0625.vhdz --mountpoint /media/ASUSUX51VZ`
Loading FUSE kernel module...
Starting VHD background process...
Waiting for background process to become available...
Mounting...
Mounted successfully.

This is fine, but how to fix the backup image mounting so that is works via the web interface?

And my second question, Is guestunmount mountpoint correct command to correctly unmount the backup image mounted with urbackupsrv mount-vhd?

Urbackup server version: 2.4.13
Server OS: Debian 8