I had the same problem and noticed a few things you can try too:
- It loads in Chrome and not in Firefox
- One sprite loaded as a gray box (in Chrome).
After checking things, the NAME of this sprite included an accentuated letter (?).
Since C2 write the file name according to the sprite name within the project, it had got the accentuated letter.
It was working fine on 127.0.0.1 but not on remote server, so it's not a bug in C2.
I changed sprite name and it's working fine.