Almost the same problem. Joomla 1.5.15 & CB1.9.1 installed then updated Joomla to 1.5.20. CBLogin module behaving consistently bad. Module published in pos7. When user first login on FE, OK. Second time user login fails with error account does not exits. User can request new passwork via email and Login OK, second time error. Happens with all user accounts including superuser on FE. superuser can still login on backend. Superuser can change user password in CB but user can't login on FE. Superuser can change user password in "system - user" and user can login one time, second time failurs. REPLACE CBlogin module with Joomla Login module and users and login and out multiple time with no errors. Ensured that new security setting for system-user is default password (not hardened) and that Google two factor is disabled (not active).