So was stuck on this issue for 45m with a friend, the issue would always happen on turn 292. no matter what. I was at war with persia and was razing cities after succeeding in taking their cities. thought that was the issue so we tried again without razing cities and that did no difference. The only resolution we found to fix the issue was to reload prior to turn 292 and surrender the war. This didn't matter to me really since I was winning the game anyway and this was just prologing the final turns of the game lol. Seemingly the only reason for the bug was being in a war at the end of the game. To clarify at Turn 292 he was not asking for a surrender, he would do so on turn 295, but on turn 292 the window would disappear. So just being in a war is the only thing I could think the issue was caused by.

Japanese Turn 295.ctrDiagnostics (2021'08'18 @1655'39''435''')p4.htmlPlayer.logPlayer-browser.logPlayer-prev.log