Thank you for pointing me in a direction I can tackle!
The only thing I suspected might be sneaking its way into the ajax stuff is the new
Ark Editor.
So I dissabled everything connected with it, and now the page is completing its load... without going white!
When I then went into the actual video section to see the Thumbnail, it had also uploaded!
However, in the Activity stream it seems to still be pulling the previous image.
I assume this is a cache issue, and hence not warranted the same thread if it turns out to be an issue.
So once again thank you!