In Drupal 6.22, after installing 'User Relationships', APK , Content Profile and some related social modules. Then upgrading the server (Ubuntu), ans saw that private messages stopped being received by the users.
I have disabled UR, APK et al. and updated the database, but it had not fixed the problem. I even dropped the whole database and used a backup from a time prior to adding social modules. No curing effect.
The server still sends out emails and I am pretty sure that privatemsg permissions are set correctly, so that users should be able to send/receive privatemsges.
This might be clue: The users can receive privatemsg from debug (privatemsg test module), but not from other users. Not sure what does this imply.
I have really ran out of ideas and appreciate your hints.