This error might occur if a username has a space in the text, like "joe bob" instead of it being "joebob", dolphin uses mod rewrite to alter the full url to a smaller url which is user frinedly, if there arent any spaces in the username then make sure you have the apache module loaded mod_rewrite in the configuration of apache. If you load the profile and see in the url of the browser an address like this:
"http://www.mysite.com/joe20%bob" then it could be one of see more the two mentioned problems.