login from comment reply not working

I am experiencing a problem with the login procedure from the comment reply or comment add links. Basically if you try to login from the link to add a comment at the end of the node you can login no problem but you get a "page not found" message concerning the comment form.

The error message looks like this:
Luogo http://mysite.org/subdir/comment/reply/NODEID%2523comment_form
Referrer http://mysite.org/subdir/user/login?destination=comment/reply/NODEID%252...

I figure that there must be some hack to be done but dunno where...