I have this happen often when I have other programs running at the same time, especially my cloud backup, sometimes nothing is running and it still happens.

I generally just walk away for a few minutes and it resolves itself, if its been awhile since youve restarted your computer, that might help as well.

And Ive found that it happens more often as HB becomes filled up, so I only transfer in my clients when I start working on the return, not all at once, keeps HB uncluttered.