Error: "Couldn’t start the internal web server."

I just upgraded from Rapidweaver 6 to 7.3. When I go to Preview mode, I see the following error:

Couldn’t start the internal web server.
Please restart Rapidweaver.

After a minute or so, the page does load normally. But restarting Rapidweaver does not solve the issue. Who knows what’s going on here?

I have the same Problem, but with the latest Version of 8 …