The mod Complex Death Messages (0.3.3) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event complex-death-messages::on_entity_damaged (ID 114)
complex-death-messages/control.lua:72: attempt to index field 'player' (a nil value)
stack traceback:
complex-death-messages/control.lua:72: in function <complex-death-messages/control.lua:45>

Ooops
Simple mistake, simple fix.

I've uploaded version 0.3.4 with a fix.

I had a filter on the damage event for characters who died, but I forgot to check that the character actually had a player attached to it.