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Maybe related to #12287 ?

DF version: 50.07 (note, world probably originally started in earlier version)
Win 10 home 64bit

No mods, no altered raws.
100 year old medium world, default embark size, played ten years in one fort, retired.
Now whenever starting a new fort with the same civ as retired fort, it crashes.
Freezes on 'preparing map'.
Does not crash when starting fort in different civ.

I have seen some similar reddit threads so maybe not just a 'me' thing, eg -
https://www.reddit.com/r/dwarffortress/comments/10da5q1/ctd_on_embark_2nd_fort/

People seem to be getting around it by deleting files out of the save and generally end up best case playable but losing all their previous forts physical construction although the site remains, but would rather not have to do that. Uploading in case this may be an ongoing issue, possibly with worlds changing version between starting a world, then starting a second fortress?

Note - zip contains saves from both before and after retirement.
Region 3 is the world crashing when trying to start a new fort.
Bonestour is just prior to retiring the first fort. Had that save so thought I may as well include it.

(PS 50.07 isn't available as an option in the DF version dropdown in this file repository upload screen).