Unable to connect to remote server == 500?

On checking of http status of an url list i get sometimes for some urls the status "unable to connect to remote server". What does this alert mean? The 5xx server error? Like "internal server error"? Or something else?
thanx

Are these errors random or for particular urls? If latter, which urls?

There are random urls from a given sitemap. I guess it could be a server bottleneck, but can't reproduce any error or misbehavior in browser.

This is what the server returns.

It the next version we have added a throttle limiting the HttpStatus to maximally 30 requests simultaneously. This might solve your problem.

https://en.wikipedia.org/wiki/List_of_HTTP_status_codes#5xx_Server_Error