If you are unable to connect using Forte's Agent, and you're sure that you're using a valid username/password and the correct server address and port then you should check the following.

Sometimes, for reasons unknown, Agent's .ini file gets lost. The solution is to copy over an old copy of the file. Reinstalling the software does not seem to replace the .ini file. 

This solution was provided by a customer who Google'd the issue and came up with a solution.