An update regarding this topic.
I forced a skip on the base defence mission by shooting down the UFO that was heading to the base base. Played a bit more - 25 days. Shot down some UFO's, did one mission, crafted some gear. All fine.
Now the game just crashes when reaching Day 257 10:44:31-ish. No message about anything, simply crashes.
Save game attached.
user_day_256_crash_2-44.json