I've got strange problem with rss feed. It shows invalid links to posts. After main domain address there is no '/' (slash sign) and it's moved after index.php

So the address looks like this:

http://forum.obywatelskiolsztyn.orgindex.php/?id=28

And of course should look like this:

http://forum.obywatelskiolsztyn.org/index.php?id=28

This is address to my rss feed:
http://forum.obywatelskiolsztyn.org/index.php?mode=rss

Any idea?