Sometimes webmasters are strange people. Obviously the webmaster of that URL is a little bit restrictive against all kind of crawlers. For example he does not accept a user agent string like 'Sphider'. Just change the according Admin setting for example to
Mozilla/5.0
Afterwards also a crawler like Sphider will index that site. Hopefully nobody tells him . .
A good example that not always the own server does have a connection problem whenever 'error 500' messages appear. Often discussed as the reason for this error message.
Tec