This error is most likely caused by a delayed reply to ajax request.
We try to add failsafe mechanism to such requests, but it's possible that in few places we missed them.
I found the one in translations and fixed it. It's not a critical bug fix, but if you prefer I can provide a fix for it (the error is not causing any other issues than the popup you see).
Can you please provide more details on when that happens? In particular - what screen are you leaving when this happens?