I was trying to customize project portal by dragging the required web parts to the page.
As a result I got an error message indicating "An unexpected error has occurred".
After some research, I found this Microsoft KB.
It appears that this problem can occur after editing WSS portal or template with FrontPage or some other editor. In our case, we used customized template. After installation of hotfix (no reboot required), problem was gone.
It's a shame this kb is not open to everyone. You need to contact Microsoft to get it, probably because it has not been tested thoroughly enough.