It doesn't depend on the filesize of my upload, I always get the white page if I upload something to any website (youtube, rapidshare, whatever).
No, I use the default lslb settings:
Max Request URL Length (bytes): 4096
Max Request Header Size (bytes): 8192
Max Request Body Size (bytes): 10M
Max Dynamic Response Header Size (bytes): 4K
Max Dynamic Response Body Size (bytes): 1M