Reference Wipe shows black

Hi!

I've encountered a problem that I can't seem to understand. I'm trying to create a Powergrade album, so I press "Grab All Stills" for my selected timeline, and the thumbnails show up, but whenever I try to use Reference Wipe, it won't show the still. It's just black.

I've had this happen before, but before it has actually made sense, because I've either changed the Gallery Stills location or it has run out of storage space. But this time, that isn't the case.

To make things even more confusing, grabbing stills to a Still Album works perfectly fine. I can wipe back and forth until the end of time.

And if that wasn't confusing enough, older Powergrade albums that I've used suddenly behave the same way. In other projects, from other disks and I just don't understand where the problem occurs.

I'm using the 14.2.1.007 version on a late 2013 iMac on a Sierra 10.12.6 system.

Does anyone know what the problem may be?

Thanks in advance!