Cannot start after the new Summer Sideshow Event d/l. Possibly by AV

After d/ling the SS update for 2017 the game doesnt start. Instead it triggers a prompt saying the game needs Steam.

There is already one thread on the steam forums:
http://ift.tt/2snqKAj

Steam file integrity verification doesnt do anything.

Personally I use AVG Internet Security 2017

Apparently making an exception of the game folder or binary folder doesnt do anything.

Disabling AVG components doesnt do anything for me. The game doesnt run.

At least three other games work as before.

Cannot start after the new Summer Sideshow Event d/l. Possibly by AV