[can't import full romset]

Found the problem (at least in my case). I was using a VPN (specifically NordVPN), and that is somehow blocking access to the Launchbox URL, which is, I assume, making an API call to pull the list of supported MAME versions and silently failing. It worked once I realized this and paused my VPN. Hopefully this is helpful for someone.

[can't import full romset]

Sorry, but having this exact same issue. The "Next" box can't be clicked, and the "Select romset version" dropdown doesn't appear. Editing the path or browsing to another directory doesn't make a difference.

EDIT: Including screenshot. Also tried to add a trailing "\", but that didn't help.