I get that a bug is unthinkable and esentially impossible. However, I wonder how many users you have using the lync editions of VVX 500 and VVX 600's with base mode set to 'generic' and trying to register to two sip servers simultaneously. To test, I just did the same on a VVX 500 and it exhibits the same behavior. Without change any other setting other than re-entering the password, if I enter the password on Line 1, it comes on line and Line 2's sip server starts recording 'wrong password'. If I then go to line 2 and enter that password, line 2 comes up and line 1 fails to register.
Both phones have the same firmware. Is there anything else I can try?