[2021] CK3 crash on Iron Mode at specific date

If it happens reliably in a particular save at a particular date, it's pretty clearly a game bug so Bug Reports would be the right place for it.

Then just put that campaign aside, and try it as you see new patches come out - typically a clear crash bug would be fixed in the next 1 or 2 patches, depending on release deadlines.

Sorry for that problem!