The mod Complex Death Messages (0.3.2) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event complex-death-messages::on_entity_damaged (ID 112)
complex-death-messages/control.lua:54: attempt to index field 'cause' (a nil value)
stack traceback:
complex-death-messages/control.lua:54: in function <complex-death-messages/control.lua:45>
due to rampant acid

Oh my goodness that was a simple mistake.
I hadn't realized that the cause of an on_entity_damaged event could be nil. Added a quick nil check to fix it.
New version with change has already been pushed, might be a minute for caches to update and it to appear.

Truly a tiny change:
https://github.com/PennyJim/complex-death-messages/commit/569f47efd8c37e6d2442db8b9b24c622cf4bfffa