Started getting the "BetterTouchTool is capturing your screen" message

I'm not using any Notch Bar features. The only recent action I added was "Custom Move / Resize Window", can it be related to that?

No, that action wouldn't be related. Maybe check in the "Recently used" section

Thanks. I just checked, and I only have keyboard actions, trackpad/mouse gestures, and one key sequence.

Maybe a screenshot action? Or the window switcher action? Or the "Pin to Float On Top" action.
Are you sure your notch bar section is empty?