This just started happening to me about 8 hours into a saved game. The diplomacy button (the one that pops up in the top left) was working correctly during my entire game time. I click it and it would bring up the proper window from the visting trader asking what items i'd like to request (and how much of a premium i'd be paying).
As of now, the diplomacy button will appear, but clicking on it just causes it to dissapear and nothing else occurs. This has occured about 3 or 4 times now.
Pre Bug Report Submission Update (lol):
I wanted to check a few things before I submitted this report. Noticed a bunch of turtle amulets that werent unloaded from the depot. I destroyed the depot and rebuilt it. I eventually got the diplomacy button pop up again, clicked it, and all is working properly again. Destroying the previous trade depot with the amulets in it (and rebuilding a new one) appears to have fixed the issue for me.