that .htaccess file looks fine to me. I’d say there is some configuration error in the host. As far as I know using suPHP shouldn’t matter; .htaccess files are Apache only, PHP’s got nothing to do with it.
It looks like you might have been “escaping” (telling the regex engine to take it litterally) the last A, which you don’t really have to but it shouldn’t hurt.
So I’m not too sure why it’s solved either, but let’s be glad it is [solved]