Fatal error, online coop mode

Hi everyone

I've just beaten the game in coop online mode two times in a row and the game crashed just when I activated the last teleporter after Providence. It crashed on my computer and not on my partner's. I was hosting but he was able to go through the credit's and unlock his stuffs, each time.
Here's the error
___________________________________________
############################################################################################
FATAL ERROR in
action number 1
of Step Event2
for object oEfFlies:

Unable to find any instance for object index '142963' name '<undefined>'
at gml_Object_oEfFlies_Step_2
############################################################################################

Two times, same fatal error : is it a known problem ? Is there some way to fix it or do I have to wait for an update ?

EDIT : I just finished the game again online and it didn't crash. So far, it crashed half the time (2out of 4)...