Build 1.2.1
20163.2582359
*shows as modded but no mods running)
As others have mentioned
in this linked post, multiplayer games cannot be resumed despite save files being available in the correct location. The option to "Resume as Host" shows as disabled when running the game after an update at the end of April.

There's a workaround which involves running a previous beta but I've found no solution for master.