Here's a list of other forum posts that describe the exact same problem. The ruler seems to be old (very old) in most cases, at least when they write about the ruler age, and the game just crashes before the death window pops up. I haven't see anyone try abdication work successfully, hence me reporting this bug again here.
The game DOES NOT CRASH if I abdicate (through a self-caused vassal faction war and instant surrender) before that date and move over to play with a new ruler. Then, as expected, the previous ruler dies on 7 April, but the game goes on, since I already am the new ruler.