I'm getting this error as well and am confused as to what's causing it. There are no 3rd party plugins being used. Only Keyboard, Gamepad, Mouse, Arr, Audio and Webstorage. I'm exporting using r117. I am using a local webfont, so I have some fonts and a css file in there as well. Could that be the cause of the problem? I don't see why it would, but maybe?
EDIT: I removed the fonts and css file, and changed the set web font events to load them from Google instead and the upload now works, so it would appear Scirra currently does not allow local web fonts for games being submitted.