The readme is large indeed, but I didn’t edit it recently and it was saving without issues before. Maybe I have too many uploaded files? I didn’t delete the old versions of the game - and if it won’t break anything now, I can easily delete those.

Yes. I’ve deleted a few files and the problem is gone. Thank you! So it was just too many old versions hanging around (I suspected that, but was worried that I can break it further if I start deleting something). I guess making a more clear error message in this situation may be a good idea (e.g. add “This may happen in case there is too much content on the game page”).