When I try to boot the game with Hovercrafts also enabled I get this message: "Failed to load mods: Error while running setup for entity prototype "slowncheap-assembling-machine" (assembling-machine): next_upgrade target (assembling-machine-1) must have the same fast_replaceable_group (assembler != assembling-machine)."
Frankly, I have absolutely no clue what this means, so I would be very thankful if you knew what was going on.

Hello - Sorry you experienced this. I tried downloading the Hovercrafts mod and tried to recreate the error and was unable to. It appears another mod is changing the group/category of the vanilla assembler machine.

If you could provide a full mod list I'll try to take a look and fix the issue! -Thanks