Yayyy - Thanks Tim
This now works.
Uploads without any 'red box' warnings, but get the friendly 'yellow' box notifications.
Though one thing I do notice, within the notifications, the following line
"Auto detection of stage dimensions failed: Unable to spot stage variable."
It does not appear to affect the upload and not 100% certain on what it means, but is it possbile that your script carries over an older reference to the 'stage'. In earlier Previews of Edge the stage reference was lower case 'stage'; in the final release (v1) it is now upper case 'Stage'. Just a thought and may help with the above message.