I’ve been able to reproduce this bug by doing this:
1) The enemy has begun to retreat (Although the dialogue pop-up has not shown up yet).
2) I end all my mercenaries’ turns but the final mercenary of that turn round (since it’s just the last mercenary swapping his weapon or bringing up the inventory screen before the “The Enemy Retreats” dialogue pops up that causes this bug).
3) Have the last mercenary move 1-2 spaces so that he has only 5 AP, then switch his weapon (this step may not be required).
Note: Automatic end turns is turned on in the options, so this may possibly be causing this bug (I’ll try further testing).
4) Exit the inventory screen and if the “The Enemy Retreats” dialogue pop-up shows up, then you cannot click on either of the two buttons.
"It is not death that a man should fear, but he should fear never beginning to live." ~Marcus Aurelius
Game: "Characters with a height advantage against their opponents are harder to hit"
Me: "That's not true, and my short axeman is living proof!"