It's not the same as your issue, but a while ago i sold an item. The buyer paid almost as soon as the auction ended, i waited until i reckon the item had been delivered; when i tried to leave feedback, i was told i couldn't. The buyer could also not leave feedback for me, we both communicated with each other in regard to this. Ebay couldn't explain it either, when either of us tried to leave feedback, it displayed:- The Auction for this item has ended.
The buyer received his item, i received my payment, both of us perfectly happy; but we couldn't leave positive feedback for each other. WEIRD!!