I have spoken to Support both live and in chat twice now regarding this issue which was first noticed around 21st August. The fix they have applied each time which I have been told is a reindex of the collections has only worked for a couple of days before failing again.
Someone has dropped the ball in the coding of the most recent cosmetic changes to the Admin & Product edit pages as the problem coincided with the change to the popup view that appears when you search for available collections in your store.
Frustration doesn't even begin to describe my feelings about this glitch, especially as it has happened in the same month that our store fees jumped from $179US to $299US.
Seriously considering moving platforms if there is no fix soon.
Had enough of these sorts of problems on the eBay platform over the part 17 years and quite frankly I am over it.
I too contacted the LIVE support and they fixed it immediately and it lasted until I hung up the chat and then it no longer worked. I emailed them...they fixed it again and once again it was fixed for about 2 minutes! They say they are escalating it now to someone else.. I am setting up a BRAND NEW store so I am having real doubts from the get go with this platform! I've spent so much time entering products I don't have time to start over! ugh!!