Cannot log frontend but backend is OK

Thanks for your time. Still, the fact that resetting manually these users password in backend always fix the problem makes me think that it is a CB problem, not a third party one.

Next time a user is blocked, I will try to take control of his profile by changing his password hash in joomla's database and make several tries.
For now, this problem is still open and I am looking for further developments.

Best regards
Antoine

I would need access to an account that's doing this for me to investigate it further as I'd need to install debug builds to output debug logs to see where the process is being interrupted, but given Joomla's forgot login on frontend isn't working either it suggests something in Joomla's core user flow is being interrupted on frontend.