Ashley, the problem was not the itch.io server. It's a C3 bug, and he causes the problem on Android, too. The itch.io founder brought me on the right track.
C3 doesn't handle the local text file creation correctly. When you create a new file or rename an existing one to "MyFile.txt" C3 actually creates "myfile.txt", while maintaining the intern reference "MyFile.txt".
In Windows this doesn't matter, but on Android or the itch.io server (they are case sensitive) the AJAX-Call goes down in a ball of fire.
I updated the bug report so you can look into that.