Login was - and remains - turned off under Captcha | Legacy.
It's possible the Joomla user component captcha is being applied to login, but I believe it's only for Joomla user component registrations and forgot login. You can try disabling it within Users > Manage > Options under "User Options" set "Captcha" to "-None Selected-".
I'm not sure where else login captcha could possibly be enabled unless you've a 3rd party plugin trying to verify captcha after login. You already have CB AntiSpam login captcha turned off so something else is certainly trying to verify captcha and since you've Joomla's global captcha set to CB AntiSpam it makes it seam like CB AntiSpam is the culprit here.
Only other thing I can think of is to just globally turn captcha off and only explicitly turn it on for CB registration via the antispam_captcha field in CB > Field Management. To globally turn captcha off go to System > Global Configuration > Site and set "Default Captcha" to "-None Selected-".