value session_data field in ci_sessions table is NULL - Why?

[eluser]moonbeetle[/eluser]
@xwero, it's very unlikely that they have the same login as the login/password is auto generated and users cannot change it.

At first glance, the problem appears to be solved. Here's what I did:
- upgraded my application to CI 1.6.1 (although I don't think that did the trick)
- replaced the session library with NG Session library
- set the session regeneration time to 600 sec to reduce overhead