OK, I think I have worked it out, its Bitdefender complaining that the HTTPS is invalid when it first connects as there is no cert so Bitdefender goes into defense mode, says the certificate is invalid and asks if you want to create an exception and continue.

If you agree to the exception, it goes to the login page and when looking at the logs you can see the error messages. If I then logout, log straight back in again, because the exception is in place, pfSense does not generate the login error.