Fatal error: Call to a member function loadChildren() on a non-object
 

I am getting this error on login:

Fatal error: Call to a member function loadChildren() on a non-object in /home/santaros/public_html/mod/menu/class/Menu_Item.php on line 246

It just started. I logged in as admin fine, logged out, then tried logging in as another user and boom -error.

The site is not available at all (only get error)

However, if I delete the cookies, site becomes available again but can only login as superuser - not as a mortal.

Most recently I changed the permissions for the (mortal) user and put him into a group. Maybe there is some way I set this up that is causing the problem?

I also restricted access to a webpage to users who are member of the group. Could this be the problem? (now that I think abot it, may it is..)

Any ideas on remedies?

Thanks for any help.