If you have create some brushes in 5.1-beta1 (or brushes created in beta1 from others) and was using the brush before going to beta2, beta2 will pick that brush on startup and crash.
You can launch beta1 and select a default brush, close it, then launch beta2 and see if it crash.
If that’s the case, use Brush Fixer for brushes created in Krita 5.1 beta 1 in beta1, then launch beta2 (as long as krita not crash on startup, no need to “convert in beta1, then launch beta2”, just convert it in any build)