I've found what causes this problem and it will be fixed in the next update. Until then, you can make the export work by choosing any value in Style > Buildings > Shape (Block/Off/Simple/Complex).
The city generator doesn't have a shape menu either ...? Style > Buildings just says "No Outline / Raised / Tight / No Triangles / Improved Lots", and adjusting them does nothing for this error??
Oh my god...I'm so stupid It says "block" for me so I instinctively thought "uh oh. dont click that" because of social media. So sorry & thanks for the help with this
Since the City Generator doesn't have an "import" function, there's no way to use this trick on old JSON exports (unless you still have the city url, I guess).
Is there a manual fix I could make to the code in the file to get the same results?