HI F.X.,
firstly welcome to the forum!!. did you by chance install both CT6 and CTA ....there seems to have been reported issues where one caused the other to act erratically.
aside from this is the usual advice - make sure you have the latest video drivers. Also check to see that you have the latest software versions, latest flash, latest Java .latest direct x build, ..etc
while these may not be the problem, it may help to narrow down what may be causing this through a systematic checklist approach.
I've had issues with CT6 / CTA as well, and it was a good install, just an intermittent bug when i had several animation packages running at the same time ....but we can't say thats your same issue until you've checked everything else that could cause this first.
Cheers,
☯🐉
"To define Tao is to defile it" - Lao Tzu