Hello!
With systemd the module "loop" is not automatically loaded.
root@siductionbox:~# mount -o loop home/user/Downloads/siduction.iso /mnt
mount: Could not find any loop device. Maybe this kernel does not know
about the loop device? (If so, recompile or `modprobe loop'.)
Here is the solution to enable it again. As root execute once:
echo loop > /etc/modules-load.d/loop.conf
After a reboot your mounts will be happy again!
Cheers
musca