After going to war with the Greeks I reduced their war support to 0. I still had 20 left. They proposed to surrender. My only option was to click on the force surrender button. The cancel button remained grey and non-clickable. However, I wanted to continue the war to increase my war support further and negotiate a better surrender deal. 

Not being able to click on the cancel button despite the supporting hover text specifically stating that the button can be clicked when you want to continue the war seems like a bug. 

I tried reloading a couple of times, even a few turns prior to the event. It happens every time.

Just wanted to let you know..

I now see this bug had already been mentioned on this forum. Many people seem to have this issue.

https://www.games2gether.com/amplitude-studios/humankind/forums/171-bug-report-archives/threads/41000-not-able-to-continue-to-next-turn-because-of-game-force-surrender?page=1#post-325852

Still not fixed.