If a game is saved while a coastal or island city has expanded into the ocean, the subsequent load shrinks the city back to size (as expected), however a segfault occurs whilst re-saving the game.
Upon tracing the error, it was found that cities_awaiting_private_car_route_check is saved using cities' center position, which upon loading, is no longer within the bounds of the city. A pull request for using the city position rather than the center has been submitted. https://github.com/jamespetts/simutrans-extended/pull/379 (https://github.com/jamespetts/simutrans-extended/pull/379)
Splendid, thank you: now incorporated.