Weird problem with WPMU & posting links

It seems that users are unable to post links to sendspace.com on my WPMU install.

I tried debugging at fast as the error is "Not found. Sorry, but you are looking for something that isn't here." but there is no error in the error log.

It only happens with sendspace links, so I figured it was a "bad word" somewhere. But I don't have a plugin installed to find bad words/links and from what I can see WPMU doesn't have it by default.

Anyone care to try if they can post a sendspace link on their wpmu install? I get the error as user or admin.