After updating XOOPS to 2.0.17 a few months back, I ran into a problem with certain users not able to see group specific blocks after updating. The users would authenticate, but then not see any content that was “member only”. I was hopeful this would have been resolved with 2.0.18 but it still remains an issue. The problem:

The solution:
In your XOOPS directory: /kernel/session.php

change:
var $enableRegenerateId = true;
to:
var $enableRegenerateId = false;

(Original fix documentation for 2.0.17: http://www.xoops.org/modules/newbb/viewtopic.php?topic_id=62411&forum=2&post_id=281576