I'm playing Victor OpenDev and an opponent (Teutons) has sent an Alliance proposal in Turn 144.

-> I clicked "Accept"

-> we are allied now

-> The red question mark (on Relations tab) does not disappear

-> Turn cannot be ended. Still says: Will you accept this offer?

-> Tried breaking the the alliance (being Traitor now)

-> Still cannot end turn. Question mark remains. End button still asks about the offer.

I just re-loaded the Autosave and the bug was gone. I was able to end turn, finally.

Unfortunately, I cannot reproduce and have not made a screenshot.

By the way: I clicked very fast. Diplomacy screen opened and I clicked Accept very fast.

And here is the Autosave file, but I wasn't able to reproduce. Maybe you are:

auto-save 160.ctr