When any user log out the password is lost. You have to generaqte a new one. The issue is:
1. You try to login, and fail.
2. Regenerate a new password from the joomla user menu and send to the user email.
3. Log in with the new password is OK.
4. Inmediately you logout
5. Try to log in again with the same pass in the step 3 and fails
6. You have to go again to the step 2
Perhaps the lost of the pass in when the user login, but in any case the same password can be used in different sessions.
