Thanks guys, you saved me a lot of trouble, and I'm not even playing a republic. Maybe this will help others too.
Some ~30 years into my game random crashes started to occur every other months, saving shortly before a crash did not always help, but surely sometimes. Crashes occured mostly on 21st of specific months (especially July), followed (by a wide margin) by the 28th. So probably something event system related. Thanks to you guys I took a shot in the dark and searched for republican great houses. Sure enough, the game produced a new republic somewhere along its way, governed by some lesser mayor at the ripe age of 77 without family. Using console, I killed every respective doge of that republic until finally someone with family was elected. After this procedure the game is running smoothly again, no crash for ~20 years already.
TL;DR: When republican bachelors are crashing your war party, kill them.