Suddenly not able to adjust Brightness on external Monitors

Since I got my MacBook Pro M1 Max, I've been running two more Thunderbolt Cinema Displays on the computer. This ran completely smoothly until the last update. Since the update, I can no longer adjust the brightness on one of the two monitors. I've tried all sorts of things, including tools like “MonitorControl” (which worked fine at first). Monitor 2 is now permanently dimmed (to approx. 15% brightness).

That's really annoying.

Any ideas on how to fix this?

Kind regards

Andi