Valid trade disappears next turn

[Donny Brook]

First of all, I have posted a thread on the tech support forum because I'm having trouble uploading my .sav file...sorry about that. Anyway, I make a valid trade of techs+lux for one lux, end my turn and then the trade is no longer in effect on my next turn. Neither side ended the trade via declaration of war, etc. It's a harbour-to-harbour trade and I can see that we both still have harbours on the subsequent turn.

Edit: I reloaded, made peace with my one and only enemy and trades were still good. My enemy must have been blocking somewhere unseen.