Hello,
Yes, it has to do with the 1/8th for validation, and in this case you also unluckily ran into a quirk of how it works. What it specifically does is take every image whose ID number is divisible by 8. I took a look at this particular source (my admin tools allow me to look it up using the classifier number you posted), and I believe none of the 26 confirmed images happen to have an ID divisible by 8. So, the validation set ended up totally empty.
Try picking one more image to annotate and make sure its image-ID is divisible by 8, and once you finish that, I think that should get it running.