Can't drag stacks!n

Hi folks - really strange problem which I just can’t work out! I was happily weaving a simple enough site (RW8, Stacks5, Foundry 3) when all of a sudden it became impossible to drag any stacks onto working area. I ended rebuilding the site (hadn’t done too much so far) but same issue came up. Again, dragging a picture into an image box was the trigger. Nothing works after this simple action, which makes no sense to me at all. Help!!!

UPDATE - it’s dragging my own images in that seems to be triggering it. Shut down and tried again only using Unsplash and no issue. Any ideas?