Hi all, and thanks a lot for your help. I managed to get over the situation. Here is what i have done:
-> I applied an pre-upgrade snapshot to the machine (this is a pure sandbox system, so i had no problems with this)
-> Cleaned the upgrade files directory (all of them: XML, SARs). The files were there before the snapshot, so i deleted them
-> Then, i noticed that i forgot to put the upgrade export in the directory (I don't think that this was the cause of the problem, but who knows...)
-> I recreated the XML file from solution manager
-> I re-copied all the upgrade files to a central directory on the upgrade host.
-> Started SUM from scratch (the snapshot didn't allow me to retake the old one).
This second time i runned SUM, no problem arised.
One important thing: when i was deleting the files XML, SARs, i noticed that there were more XML files on the directory than the MOPz one. I am not sure now that the problem referred to the MOPz file, because i cannot recall the log error exactly. Also i had already applied the snapshot to the VM, so I could not look at the logs again.
These other files, i have no idea what they were. But they could be the source of the problem.
Thanks