I also faced with the same problem as topic starter.
When I upgraded from 3.4.2 to 3.5 sometimes when I uploading file with Add Media on create post page – have the same message and than admin panel crash. When using Media-Add New – all media uploaded OK.
I upgraded to 3.5.1 and now I get crash even when uploading with Media-Add New! Media files uploaded, but when finished – it’s crashed.
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