It might be possible that after you changed the port a rogue BIM Server service remained running. Often restarting the machine is the best way around this problem.
Though there is something more problematic in the long run than your port issue. You mention that you have installed the BIM Server on a Windows Small Business Server and it is running an Exchange server. My guess is that the OS is also 32-bit. Your teamwork experience will probably suffer with this choice, since the BIM Server is sensitive to hard disk availability. The Exchange server at times can seriously occupy the hard disk's resources. If the OS is 32-bit then you only have 4 GB of memory, for which a substantial portion of that is also devoted to Exchange.
The performance of Teamwork on such a system will suffer periodically. You don't need to run the BIM Server on a "Server" operating system.
Ed
Graphisoft Technical Support