I'm having the exact same behavior with 3.19. I upgraded to 3.20 today and the issue persists. I can confirm rolling back to 3.18 fixes the ability to import ROMs.
When 3.19 / 3.20 are installed all my platform game folders are displayed as a UNC path rather than the relative path that was shown in 3.18 and before when choosing Tools --> Manage --> Platforms.
Importing ROMs whether it be a folder or file always prepends file:\\\ as described above. It's worth mentioning also that every time the import process starts, it defaults to "network" in file explorer and immediately shows a UNC path as the current directory before allowing me to browse to the network drive that inevitably results in a crash with again the same error described above (The specified path is invalid).
I store all my ROMs on a file server accessible by a mapped network drive (R:\). My Windows PC is domain joined to a home lab / network
