Forty-TwoFix the 'An issue with your computer is blocking the VAC system, you cannot play on secure servers' bug
I'm aware it's not TF2-exclusive, but seriously, about half of the scrims/officials I've played or watched lately have had to deal with this :<
Seeing as this is something that affects CSGO, I would assume Valve has been looking into it and trying to find a fix. Presumably they either a) have a fix that isn't ready to deploy b) haven't found the cause yet or c) this isn't something that can be fixed