While your panorama is 8000px wide and is publishing OK. The fact remains if you are only publishing to HTML5 you gain absolutely nothing having larger files. Your cube faces will be reduced to 952px (iPad) and 500px (iPhone) which is just under 3000px and you will possible lose pixel definition via the downsizing interpolation. 3000/Pi = 954.957px or 952xPi = 2990.708px
Of course if it works for you and your happy then their is no need to change. It is the way it "should" work, so we can publish to both Flash and HTML5 with one click.
My suggestion was to attempt to help DaffRider with a possible solution based on freeing up memory.
It could also be the same issue where non English language operating system is causing issues.