sidebar.php

the <?php get_sidebar(); ?> shouldn’t be causing a problem like that (unless there’s something that’s really invalid/broken inside sidebar.php), no matter how bad your css is, it won’t cause a change to, or “break” your permalinks, that being said, i’d start with:

1) check permalinks configuration via admin interface
2) check permissions and .htaccess to make sure all is well
3) go over sidebar.php to find the broken code (it’s not going to be some html/xhtml/css causing that kind of problem, that’s going to be some bad php.

let us know what happens, it surely doesn’t make much sense, i just checked the site and all appears to be working ok.