Seems to be some kind of infinite recursion with these three functions:
#6 0x0000555555703770 in improvement_obsolete (pplayer=pplayer@entry=0x0,
#7 0x00005555557c8b2a in is_building_in_range (target_player=target_player@entry=0x0,
#8 0x00005555557cb3ae in is_req_active (target_player=target_player@entry=0x0,
That's because in the buildings.ruleset Colossus is made obsolete by its own non-existence (no idea what that tries to achieve)
That line was supposed to be in reqs, not obsolete_by. I made that error when I added that line to all small wonders, already suspected that I did, because I already slipped in the line several times then. Didn't expect that to be the cause of the crash, because I didn't get an error. Had to correct it on two occasions, now it loads again
Created a meta-ticket about the infinite-recursion with requirements cases we have -> #45974
S3_0 server segfaults when starting a game with SlowScience ruleset. It loads fine.
Reported by 3plus4i in https://forum.freeciv.org/f/viewtopic.php?p=106610#p106610