Debugging Backendforwarding and UP status

[Sebastian Fohler]

Is there some simple way to find out why I get this error from my 
haproxy cluster?

 503 Service Unavailable

No server is available to handle this request.

It looks like all my backend servers are down. Even in pools which are 
shown as up in my stats.
How can I debug that sensible?

Thank you in advance.
Best regards
Sebastian