i think i found a bug in the import rom section, when the files to import are a huge number; here what i've found:
I've a Sega Saturn folder with 21185 games in it, i've set Mednafen as Emulator then I renamed the folder with the rom files as "Sega Saturn" and i moved it into the LaunchBox/Games folder;
i started the Rom Import routine, choosing "Add folder" and selecting the Launchbox/Games/Sega Saturn that i moved before.
Choosing the option "use the files in their current location", at the "Ready to Import" window, Launchbox couldn't find ANY rom to import !
After several attempts, I tried to move the Sega Saturn folder outside the Launchbox/Games, in order to easy find it I placed in the root directory of my hdd (D:\Sega Saturn), so i re-started the Rom Import selecting the D:\Sega Saturn folder and, this time, choosing the "Copy the files in my Launchbox Games folder", THIS time Launchbox found ALL the games and I could start to import them.
Please note that this 'bug' happens only when the rom files are "a lot" I don't know exactly the number but if you try with a folder with about 100 files it works perfectly in any case.
It is also quite IMPOSSIBLE to remove the extension you don't want to add at the "Ready to import" window; if you select, for example, .bin files and the press ESC in order to remove them, the program "freeze" for a very long time. I waited 4 hours in this state and then i had to force quit Launchbox.
I use Windows 10 64bit version 1909 build 18363.535 on i7 3,40 Ghz and 8 Gb Ram.
I hope this can help.