Fixed following Point 4 here (format "/System", then reinstall CM7 and gapps).
(My error probably was that i rebooted the system between the 2 installs.)

EDIT: now i'm having an issue extending the internal storage: I've installed s2e, created the ext3 partition and moved the apps, but after the reboot all the apps are disappeared...

EDIT2: fixed by reinstalling and using Mounts2SD instead of s2e.