Hello again Alexander,
I was at that thread earlier, as the same error was popping when I was trying to set a cron job for email notifications. That specific answer from this thread which you suggested, didn't help, as layout was chosen.
However, after doing more search, I found a suggestion to change this in a file named frame_inc.php. And voila, the error disappeared, except that when I try to register, the window which should notify me about Facebook application getting following data, etc, disappears and nothing further happens. The browser goes back to main page and there is no new user added.
What's interesting, earlier it was different, the window stayed open, the problem was the error. Now, it varies - sometimes, for some test-accounts on Facebook, the window stays open, but after confirming that I want to give my data, window disappears and nothing happens.
Thanks again,
Artur