Commented Unassigned: AsyncFileUpload (Toolkit ver. 7.1213)+ Opera browser (ver. 12.17 1863) = errors on big file upload [27723]

I created page with asyncfileupload inside update panel. It works on IE, FF and Chrome, but in Opera i get JavaScript Server Response Error: 'Unknown Server error' popup every time when i try to upload file with size more then 2-3 hundreds of KB. Help me to find solution, please!
Comments: here is a sample of project