a 500 error is a server error, for example a typo in the php code or a server misconfiguration.
Some server respond with a 500 error on timeouts.
The problem is that a lot of possible issue's are hidding behind the error 500's. So I need you to digg into the error log of the server to find some more useful message. The location of the error log depends on your system.
For zanox feeds use a starting point the CSV version of the feed with the parser set to 'autodetect' and the callback to generic_cb .
If you want to use the XML feeds the parser is either 'zanox' or 'm4n',