The two issues are unrelated. PDX never logs anything useful to error.log when it crashes any game.
1. For economic, what you're probably seeing is some trigger or effect or literally anything relating to economic ideas which asks "Am i <someTAG>" where someTAG probably doesn't exist due to it being a conversion and the tag is undefined. It's hard to pinpoint this seeing as you see this issue, but I've never seen this before, and it changes nothing of consequence.
2. As for crashing, that's a persistant problem across many of the games which is next to impossible to debug. Some (very likely vanilla) reform, some effect, some decision somewhere, with a MTTH of something (so it's never any single date) will cause the game to crash because something gets divided by zero or who knows what. And since PDX doesn't acknowledge bug reports related to mods, stuff like this doesn't often get fixed.
Better logging would solve a ton of these problems, but we'll see if EU5 will implement such debugging.