If you are using Windows Ink, then that’s the “problem”. Do this:
Find the file “qt.conf” (probably here C:\Program Files\Animation Paper\qt.conf). Delete it or rename it (to save it). Then launch Animation Paper again.
This will force Animation Paper to use Windows Ink. I hope this works! It should give you pressure sensitivity and everything you are used to. Please let me know how it goes.
Cheers,
Niels