The image below shows the problem. Sometimes, Photoshop doesn't show the correct images to be exported. Here you see blank thumbnails, and the names of the Artboards are from a file I just closed, not the file I'm currently in. Before I closed that file, the thumbnails were showing the images from the other file, so I tried closing it to try stopping this behaviour.
To stop this, I need to close Photoshop and open the correct file again.
I'm on Windows 10 Enterprise, 64bit.
PS version is 2017.1.1 Release.
Thanks