Going back from 3.4 to 3.3. Could be the cause.
There’s a hidden Audacity folder which persists from one installation to the next.
Users\<username>\AppData\Roaming\Audacity\
In that folder is a file called “pluginregistry.cfg”.
If you rename, (don’t delete), that file to, say “pluginregistry-old”,
then restart Audcaity, that may cure your frozen screen.
[ BTW scanning the plugins will occur when you restart Audacity,
the first time this happens it can take many minutes,
depending on how many plugins you have on your computer ].