I have been reporting same type of behaviour in the data model section of the forum but related to the dev build phase. Got no answer there.
The same type of behaviour happens even in the ImageDataObjectManager gallery i have. It has always worked fine, in local no errors, just on server, Server has been working always fine and currently it's running many silverstripe based sites.
When i try to add an image to the gallery, the opoup appears and it's all white unless for the close X button. Then, a BROWSER popup appears asking me if i want to download a file called Upload.php - of course it's 0 kb.
Trying to reload the cms, the admin page goes all white. I have to close the browser and start again. Even in the dev mode no errors displayed.

Help, please!