By default Linux allows you to mount up to 8 iso files simultaneously. For some XEN VPS nodes this is not enough.
To resolve this issue, open
and add the following line:
options loop max_loop=128
When you want to mount up 128 drives simultaneously, or change 20 into 15 when you only need 15 mounted iso files.
Once this has been done you need to reboot your server for the change to take effect.
Add max_loop=256 to your kernel line or run 'MAKEDEV -v /dev/loop'
Create /etc/modprobe.d/local-loop.conf with this content:
options loop max_loop=64