This may be a long shot, but we have had similar problems within the office. Firstly, it sounds like you have your port mapping in hand or it wouldn’t connect in the first place, but if you dont then I think you need ports 8080 and 1800 going to your server.
We occasionally and randomly get the same problem happening inside and outside the office. It came down to permissions on the Mac client. Sometimes repairing or resetting the permissions on all of the C4D installation worked. It’s very weird….
Steve Bell
http://www.archiform3d.com