Thank you, but unfortunately i already did this: uninstalled 2.81a (as the other versions are portable), cleaned the registry, and also the user directories. And now i can not reproduce the bug on any of the portable versions. Everything is working fine just as expected. Must had something to do with the earlier installer version interfering with the portable ones. That's including the crash on the attempt to restore the factory defaults (now it is working just fine).

I just tried that. Blender crashes on the attempt to load the factory defaults, yet saving the defaults (startup.blend) is working as expected. I also went to the config folder and removed the startup.blend. Which returned me a new default scene on the startup, yet the bug persists when i'm trying to add and modify the arc curve.