Toon Boom 21.0.3 does not launch on Windows 11

I have Windows 11, freshly installed. My animation studio which I work with is trying to install the 21.0.3 version, but even after activating the key, once launched, it stops or asks again for license (which is absolutely valid and legal, it’s a studio).
This is very frustrating.
What should I do now? Buy another OS? Back to Windows 10? What if I can’t downgrade?
I work with Toon Boom. I need support asap, please.