Drupal does not permit access to YaBB forum

Hi,
Long time ago I've setup my web page as regular html. When I added forum I've chosen yabb. Then I've installed Drupal and replaced html pages, but as I had no time to set up Durpal's forum module properly, I had to leave yabb.

So, now I've transferred my site to new server and www address. There I've installed Drupal 5.7. As I have no time and knowledge as how to convert yabb into Drupal forum, I've installed yabb and uploaded files from old site. As soon as I log out of yabb (to switch from admin to user), Drupal replies with "Page not found".

Currently the old site I have same version of yabb and Drupal 5.2 and there it is working fine.
For both sites I use Golden hour theme.

I though it had something to do with yabb - reinstalled about 20 times in 2 days. Checked everything with server and host provider. Not a problem there.

It is definitely a problem with Drupal because it does not permit me to open that page. I've managed to access forum once when I switched the theme from Golden hour to default Garland. But When I switched to third, I got message "Page not found" again. This time, returning to default theme did not help.

I've compared data from both servers and they match. Only thing that differs is Drupal version, one is 5.2 and the new ("Page not found" one) is 5.7.

Thanks in advance,
Renato