Always same menu item is selected.

I'm having a weird problem that I can't pin down.
First: I have set up submenu under the Primary Links. It's no more than 3 level deep. I've installed the Menu Block ( http://drupal.org/project/menu_block ) module to display the submenu.
This morning, I noticed that the same item is always selected. I removed the Menu Block module and put the default menu instead, but nothing happened. I tried to move the item, maybe the menu was always stuck on the first submenu item, but it seems to follow that particular item. I can't see any difference between the 'always selected' item and the others.
Otherwise, the menu works fine, it points to the right pages with with correct url aliaes and everything.
J.