Hi,

I've been trying to solve this problem for over a week now.

I have a Drupal 6.19 company website, with 11 languages and over 1400 total links
in the menu_links table. Whenever I try to add a link to primary links, Drupal starts to
behave strangely: I cannot edit or add any nodes, with an error message "username admin
does not exist" - But I have the admin user and I can login with it. If I then delete a menu
item from primary links, everithing is back to normal. It seems somehow a limitation.

Here's what I already did:

Rebuilt permissions, deleted cache, forced rebuild of navigation links
Checked menu_links table throughly, searching for ivalid items
Deleted some orphan menu items that had no parents
Corrected some menu items that were internal but somehow were flagged external in DB
Made a local copy of the site: the issue is the same
Updated Drupal 6.19 to 6.26
Updated all modules to their latest version
Tried to disable ALL modules except menu module: issue persists until I disable menu module too
Checked that all items in menu_links appear in Drupal UI
Increased PHP memory limit from 128MB to 256MB

None of the above solved the problem. This issue only occurs with primary links. I can add menu items
to secondary links without triggering this problem.

I searched the web for a solution, but all I can find was a similar problem with no answers. I posted this
question in the Drupal forum, Stackexchange and Stackoverflow but nobody could solve this problem.

If there's anyone who can really help me with this, I'm gonna put him on a pedestal and worship his name...

Thanks in advance for any clues,
Balazs