Error when subscribing a new user in VM 3 after Joomla migration

[SteP[IT]]

I have migrated a site from Joomla 2.5 - VM 3.0.9 to Joomla 3.4.2 - VM 3.0.9.4.
Now the VM user registration from cart doesn't work anymore (the same was working in J2.5 28/VM 3.0.9)
The subscription form submission aborts giving this error message:

Code Select Expand

Salvataggio fallito con il seguente errore: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ' 2)' at line 3 SQL=INSERT INTO `j3vm3mmc_user_usergroup_map` (`user_id`,`group_id`) VALUES (, 2)

The query lacks the $user_id value, but I can't understand the reason...

I have also overwritten VM3 and Joomla 3.x files, taking them form a new J3.4.2/VM3.0.9.4  installation where the subscription was working normally, with no success.....

Could someone give me a hint to locate the issue?
I'm noit using any VM override, standart ATOMIC template.

Thank you in advance for any help.

Regards