I'm having this exact problem.. the Control Center site was working, but since an IP address change I haven't been able to access with an old or new accounts.
I've made the change of the IP address suggested and restarted the Labtech server and done an iisreset, but still no joy. The user definitely has Web Access flag set.
Anything else that might be stopping this?
-EDIT: Actually ignore that, I managed to solve this by using the internal IP address in the suggested location.. rather than the external IP address (which it was set to before). Might be something specific to our setup.