do you have some form of transparent proxy or caching system that is misbehaving in front of your Moodle web server?
Turns out, after being told the proxy was not a problem, it was. Thanks for your help.
It's happening to us too!!!
How is it possible?? We've got this in mdl_log:
The user 82485 login with his correct username and suddenly he was other user, 49432...
Our Moodle version is 2.2.9, how did you solve this issue?? what was the wrong with your proxy??
Thanks a lot!!