individual user access control lists do not work

with enabled access control settings per node, any attempts to add users to user access control lists fails with the following message:

* warning: Invalid argument supplied for foreach() in /home/www/includes/form.inc on line 1344.
* warning: Invalid argument supplied for foreach() in /home/www/sites/all/modules/acl/acl.module on line 123.

Running Drupal 5.9 on RHEL4 (php 4.3.9, mysql 4.1.22).

This behaviour is similar to nodes that were created prior to switch to 'per node access', or after. It is also similar to different node types: built-in, like story, module-provided, like webform, or created manually with CCK.

Any help points are appreciated!