I am having the same problem, so instead of starting a new topic thought I would append my information. I am on:
Joomla 1.5.26
CB: 1.8.1
pHp: 5.2.17
1. Cannot add users from Backend
2. Can register users from front end, but can't edit entered users information from back end admin panel.
3. Users profiles entered earlier (perhaps before 1.8.1..but not sure) can be edited and saved.
In all instances where users cant added or edited the save button returns me to the CB home screen.
Looking for help in areas where to begin troubleshooting.
Thanks-