This sounds strange as the members and the request pipline doesen't have so much to do with eachother.
Is it a clean install? Are you using any custom url rewrites? I just did a clean install, created a content item called "Test" and set the umbracoUrlAlias to "Glenn" wich worked and still works after adding a new member called Glenn.
Are you displaying profiles of the members? Ie. http://www.mywebsite.com/memberName/
Where "memberName" is the name of the member? In that case this is probablt why you are having trubble.