I was adding a dimmer to an existing screen full of dimmers & on/off switches. I started by trying access image files to use for the control. I was able to see all of the images in my DropBox account when I pressed the DropBox button to select an icon, but the image didn't get assigned to the control. Then the error started popping up & wouldn't go away.
I'll try to replicate it if you want to see logs. In the meantime, I was able to add the control with just a ring.