Gosh, that was my fault. I finally tracked it down to a changed field name in main.inc.php ('mail_field' in beta2 to 'email_field' in rc1), only to find that it is clearly stated in the upgrade instructions. I missed it due to the fact that I haven't been using LDAP in beta2 (so I just skipped this piece of instruction while upgrading) and later, when I decided to use it in RC1 it bit my ass (due to the outdated config file).
Sorry for the confusion,
r.