Below is a link to a saved game in DropBox that should allow you to replicate a freeze on the turn of "Unknown Civilization 15".

https://www.dropbox.com/s/l9ws0sw2sqk5h9c/FreezeOnCiv15.GC4Sav?dl=0

If you load the saved game and click on the icon to go to the next turn, a window should pop-up for a successful treasure hunt,  Then, when you close that, you should see the game show the other civilizations taking their turns in the lower right-hand corner until it gets to "Unknown Civilization 15" where everything except the music stops.

Please use this save to figure-out what is causing the freezes.  It is very annoying and I do not know any way to get past this.