Error 503 on products with allow ordering when out of stock and user logged in

[Innla]

I have a strange problem, I have error 503 in some products. After a lot of debugging I can say that it occurs only in products where i have activated the option 'Allow ordering' when out of stock and only if I am logged in.

Also I check server logs and when I access one of this product I get this error: 

503      GET /mc-04369 HTTP/1.0

(104)Connection reset by peer: AH01075: Error dispatching request to :

I don't have any idea of why this is happening, any ideas?

Thanks in advance!