You should try to edit your client machine hosts file. On Linux it is /etc/hosts, on Windows it is usually C:\WINDOWS\system32\drivers\etc\hosts. You should have administrative privileges to edit those files.
Make sure that hosts file doesn't contain entries with server.com. If so, comment them out. Then add a new entry of the following form:
11.22.33.44 server.com
Then go to your browser and try to log into Panel using server.com domain name.
Note that this is merely a potential workaround, and designed to check a hypothesis about the reason of such behavior.