For everyone reinstalling, before formatting or restoring the image, please REVOKE the license first, the code will be then available for reuse when you restore from backup:
/usr/local/fop2/fop2_server --revoke
Virtual Machines do not mantain same MACs, or Host ids, even after restored from images or whatnot.
And it is NOT THE SAME issue for both posts, the 1st issue is that the VM does not have a valid eth0 interface, in those case, you can create a dummy one on the host os (see FAQ), or you can specify the interface you want to tied the license to with the -i command line option. Supose you use openvz:
/usr/local/fop2/fop2_server -i venet0:0 --register
(ALL of these commands are in FOP 2.20, if you use FOP 2.11, please upgradre).