An HTTP error of 500 to 599 (although only very few numbers are actually used) is different from the more known 4xx errors.
The 4xx errors indicate that YOU as the client have done something wrong and you should modify your request, so that the server can fulfill ist - the most prominent error is 404 web page not found (more precise object not found).
A 5xx error on the other hand indicates a server error. That means, you probably did everything right, but the server is unable to handle your request.
With 4xx errors you could continue issuing your request till the end of all days, it won't work. With 5xx errors it could, at some point, work - examples include heavy load on the server typically resulting in a 500 (internal server error).