I found a few threads on this (
link,
link and
link) but nothing definitive.
Maybe try deleting the savegame folder and then recreate it and download the turn again, try disabling all mods and quitting the game and then relaunching and trying to open the turn, and try resending the turn a couple more times, and maybe we'll switch to a different email address as well.
If none of that works then the only two options are to rollback or for you to stale (whomever you're at war with shouldn't take advantage of the stale and there should be essentially a one turn ceasefire) and hope one of those approaches fixes the problem. The first of those links indicated a rollback didn't solve the problem so I don't know if that will actually work (rollbacks do involve some risk since if people don't delete all their old files you can get all kinds of problems).