What is "Err 13" on a Mamiya 645 AFD????

[tirediron]

I was out doing some shooting today, and decided to pull out the old 645 to shoot some black & white for potential big enlargements. For some reason the old girl, who has worked flawlessly ever since I got her, started blinking "Err 13" at me. Everything seemed to work correctly, the shutter fired, the film advanced, and even rewound when I came to the end of the roll. I don't think it's anything especially serious, BUT there's nothing in the manual, and thus far an Internet search has only turned up other people asking what certain Mamiya error messages are. If anyone has a full list of them, I'd really like to know.

I've e-mailed Mamiya, but I'm not holding my breath for a speedy response.

TIA