I am not familiar with Ubuntu server's install, but am with desktop Ubuntu. Were you prompted for a user name and password during the install, and is that what you are using to log in?
The only thing I can think of is the something got set wrong during the install.
It would not hurt to have a live CD. I don't know whether or not Ubuntu server's installation media serves as a live CD. My favorite is Knoppix. I'd try to look at your logs and find your installation files. I'm looking for the exact location of those, and will post the answer back here when I find them. Basically, those files would tell the Ubuntu launchpad people a lot if you decided to post this as a bug there:
Here is the location of the install files:
I have not done an upgrade yet, so my dist-upgrade directory is empty, but my installer directory has files in it.
What's the Ubuntu server version?