"There has been an error saving your changes"

Whenever I tried to edit the Fixer theme using the Upfront customization page, it gives th "There has been an error saving your changes" error after I make few changes.
Fore example, I changed two of the background photos, delete another two, and save. That´s ok. When I add two more and delete two more of the original photos and try to save, it gives the error. Same thing when I tried to edit the front page text.