I didn't get any error. It's just that I can't change the post thumbnail in the gear icon (post settings).
The thumbnail is the first image by default but I want to set my third image as the thumbnail (since first and second is divider graphic). When I tried to change it, I tap on the 3rd image but nothing happened. There's a sound feedback (this is due to my phone settings btw) when I tap on it but that's all.
So I ended up removing my divider graphic so that the image that I wanted becomes the thumbnail.
Thanks for trying to look into this matter.