What exactly did you install? Vegas Pro 21 is not bundled with BBC, but with Boris FX Primatte Studio. Do you mean Boris FX Primatte Studio, or an BBC version?
Boris FX Primatte Studio 2021.5 that is bundled with VEGAS Pro Suite 21. It worked until I uninstalled old version of Primatte studio and installed new version.
What exactly did you install? Vegas Pro 21 is not bundled with BBC, but with Boris FX Primatte Studio. Do you mean Boris FX Primatte Studio, or an BBC version?
Uninstall VEGAS and primatte completely. Reinstall VEGAS, then reinstall the new primatte. See if that helps. Something may have been corrupted when updating the old primatte to the new one.
It worked until I uninstalled old version of Primatte studio and installed new version.
But that looks like, as if the new version has worked fine in the beginning. But the deinstallation killed your system.
Sounds like it would be necessary to know exactly what has happened here during the deinstallation. Or - since you cannot know - that it may be the better choice to re-install Windows again.
When I initially installed VP-21, it continually crashed on launch loading during the splash screen display. The finally found the error report which indicated it was a Boris plug-in factory or something like that. I uninstalled the plug-in and all is good. The plug-in was from a VP-16 install.