Since I happen to be in a related area of code at the moment while working on GSS3.0, I've just inspected the code which manipulates the LastImage table and I have a theory as to the cause of the database error (the pop-up describing the database error is indeed the cause of the console stalling and tasks not completing, but in order to resolve this it's necessary to determine the real cause of the database error).
The LastImage table has a primary key column "Identifier" which was meant to hold the same value as stored in the Machine.Identifier column, but it appears to be too short (only 30 bytes instead of 80). The code attempting to adjust this table is doing a query to see if the value already exists, but if the identifier value is being truncated then this may be the reason for the primary key constraint violation.
The content of the Machine.Identifier field is a list of the MAC addresses of the network adapters in the client machine - to overflow the 30-byte column size a client machine would have to have more than 4 network adapters associated with it, which would be quite unusual. In general, the only reason this would happen is due to something like VPN software which installs fake software network adapters.
It's likely in this case that the problem will be associated with a specific client machine; to confirm the diagnosis, if you can get this to occur again, before pressing "OK" on the error dialog, take a copy of the server internal logfile (in "C:\Program Files\Symantec Ghost\ngserver.log") and get in touch with me at the gmail.com address in my forum profile - that logfile contains trace information about the client/server communication which may help to confirm the diagnosis (along with the identity of the associated client and possibly why it has an unusually large number of network adapters).
If this is confirmed as the underlying trigger, then the most reliable resolution will be a small code change to the main executable on your GSS server. Once we can confirm the cause I can submit a potential fix to our build process in order to make you an update.