http://www.episerver.comwill cause the error.
http://www.episerver.com/will work fine.
http://www.episerver.com/devforumwill work fine. However, once you have created a "buggy" page you will have a hard time editing it - just as you described. Reload/refresh your edit mode, delete the buggy page and create a new one - making sure you add that final "/" to the domain URL.