Photosmart 6510: No-paper error but paper is in both trays

When I give a print order, my Photosmart 6510 E-AIO shows the message that there is no paper in the tray, but there is really enough paper (and not too much) in the main tray and in the photo tray. The printer is not 2 years old yet. I already tried the HP Print and Scan Doctor, but no result. I get the same message when I try to print a test report directly from the printer, so it's not a problem inthe communication with the pc. The pc runs on Windows 7.