Same problem here on Illustrator 23.0.3 running on MacOS Mojave.
It works for a limited amount of times when I restart the illustrator or create a new brush.
Just did some other tests and I could change the settings an unlimited times only if I create a brush and don't change the name. When I finally change the name after a few setting changes I couldn't hit OK. But if I click Cancel and come again to edit, I can change every setting normally and click ok only if I don't change the name.
Same problem here on Illustrator 23.0.3 running on MacOS Mojave.
It works for a limited amount of times when I restart the illustrator or create a new brush.
Just did some other tests and I could change the settings an unlimited times only if I create a brush and don't change the name. When I finally change the name after a few setting changes I couldn't hit OK. But if I click Cancel and come again to edit, I can change every setting normally and click ok only if I don't change the name.