It definitely is in the zip file, here's a fresh download from itch.io on Windows:
Logic World uses the same architecture for singleplayer as it does for multiplayer: when you play in "singleplayer", it's actually spinning up a local "integrated server" and then connecting to that. This issue doesn't look like a problem with connecting to the integrated server - rather, the integrated server process is ending for some reason. Just spitballing here, but some antivirus software might be killing the process.
Can you check the logs folder for a file called log-[timestamp]_server.txt? That file should contain more information about why the server closed unexpectedly.
Apologies for the delay getting back to you, I started typing this comment two days ago but forgot to finish and submit it.