Probably a bad .htaccess file.

In MOD Robot posts, the Download file link seems to be broken, atm. It'll take you to this page:

http://www.phpbb.com/mods/db/download/3224/

In turn, you'll be redirected to this page:

http://www.phpbb.com/mods/mods/db/index.php?i=queue&mode=overview&contrib_id=3224

Removing one of the two mods/ fixes the problem.