@fabiobr , i believe you activated the 2nd opinion scanner from WD. I just tested on my system right now. WD was disabled and Kaspersky activated. I enabled the WD scans, and i got exactly the same result as you did in your first post. Try to reboot and disabled it.
It has happened to me too with Comodo Internet Security. Sometimes the AV can't register in Windows Security Center (even if it's running well) and Windows Defender re-activates.
It has never happened with my current AV, FortiClient.
Just in case, I have disabled Windows Defender by O&O ShutUp10