Hey Leonard, I'm actually experiencing the same problem if I open the example .lvl files using the SDK Mammoth. I suspect it's related to the JSON revamp that took place within the last couple years, which completely changed how the database files are read and stored. This was never truly tested with the SDK, so we'll be looking into it and pushing a solution once we find it. Thanks so much for pointing it out!
ChopBam's post in W3d launcher - no servers listed was marked as the answer