When I had a friend join me in multiplayer starting on Vulcanus, the moment he joined I got this error:

The mod Simple Seablock (1.5.0) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event SimpleSeablock::on_player_created (ID 32)
Player does not have a cutscene active
stack traceback:
[C]: in function 'exit_cutscene'
SimpleSeablock/control/starting-items.lua:32: in function <SimpleSeablock/control/starting-items.lua:7>

Thanks for the report. I'm not really set up to test multiplayer, but based on that crash log I'm pretty confident 1.5.1 (just released) will fix it. Please give it a try, and if there are still any issues let me know and I'll figure out how to run multiple instances to actually test multiplayer.