All is in the title : a vassal ally has its emissary ambushed, the ally never resolves the attack.
Whatever I do, the turn never ends, and the game blocks.

Here are 2 saves : this one new_fun.ctr creates an error at the load, and this auto-save auto-save 529.ctr is right before turn 277 and the ambush.

If need be, here's included the log files Temporary Files.7z of the last minute when trying to end turn / load the save.