XF 1.5 Can't get on the site

When i'm not logged in, I can access my forums fine without errors, load all the pages, threads, etc, but when I log into my account, it loads the forums and everywhere I click it says, "The requested page could not be found." And in the address bar it includes /defaultsite at the end of the address. It also says Error in the tab and shows a small image of the site where I bought my domain.

When I load the address on XenForo with the /index.php it gives me a 'Multiple Choices' error.

Any idea?