Hi, yes - if that fight is from today, that looks like the very fellow. I had it happen on a previous attack as well - won the fight, went to click mug, and got the "this fight no longer exists" message.
In your previous fight (I mean when you attacked that exact player) your mug was succesful also. I don't actually see any issues in logs. Most likely this was a kind of late poll request that worked after the fight was actually finished. Attack polling system is still something from old days, so it may act like this when we have a high load. This don't have any effect on actual results.
I think that could have been it. Let's close this for now, and if it happens again, you may create a new report — assuming it didn't happen during a time when a lot more attacks will be happening simultaneously when it could happen.