I've hosted this chapter a few times, everyone else always crashes out when the barrels 'splode. I found that friends crashed out just before the boss battle in the village with the M60 (I forget the name) too.
I don't have an error report myself, obviously. Friends are playing on computers good enough to run Magicka (They've both played loads of the SP themselves, including the parts where we're crashing in multiplayer.
I'm hosting on this machine:
Mobo: EVGA 680i
Processor: Intel E6750 @ 2.66 GHz
Memory: 2GB DDR2 @ 1.133 GHz
GPU: Geforce 8800GTS 320MB
... with a fast university connection with good upspeed.
If players could join a game in progress, we could continue past it and other crashes like it. Yesterday or the day before Magicka hung when they crashed out and I moved to the next area and now it just kills them and lets me continue alone, so that's a more elegant failure
Magicka represents the most fun I have had in a game for a long time and certainly gives the most laughs to us all. Please keep patching it out of this beta state.
EDIT: And this should be in the Tech Support Forum, I think.