Creating 'local' LDAP users with feed importer

Hello,

I have imported a few dozen of LDAP-users to Drupal with Feed Importer (not with LDAP Feeds - beacuse of its 'alpha' state). It would seem totally pointless, but I had to prepare these users with special user roles in advance and they can log in in mixed mode.
When I try to sign in with one of them I got a white screen in /user page and HTTP 500 AJAX message in the modal login form. After reloading the page the user is authenticated but it's definitely not a user friendly way to log in.
I think there's a default value in 'data' field of 'users' database table which makes this havoc. Is there a way to clear this up opr should I try LDAP Feeds?