I've found a solution! Ha!

The address of the forum must be OBLIGATORY in this format:

yourdomain.sth

If You would make a subdomain like that:

subdomain.yourdomain.sth

Then Your forum will be working, but RSS feed will have error in posts link. Why? That's not my problem. My problem was, that I placed my forum script in directory one level higher than my website, which is parked under my main domain obywatelskiolsztyn.org. I wanted the forum to work properly, so I moved it to main folder. But there were also one more thing to do - I had to delete all files in templates_c directory.

That's all! Forum is working. RSS feed displays links correctly.