Okay, nevermind, I guess I could figure it out myself. For those who are interested, I'll explain what went wrong.
I read somewhere that action 16 (Reveal all map) causes reconnection errors in multiplayer. In order to create a sort of fog of war for this map, I set up a repeating trigger with action 51 (Reshroud Map). Turns out that action causes reconnection errors too.
So, to fix the issue while keeping the fog of war, I simply deleted the trigger with the offending action, converted an invisible light post into a gap generator with a huge radius and placed it in the middle of the map.
Chrono Vortex's post in Map Causing Recon Errors was marked as the answer