GTM - HTTP check through proxy

I made already a trace on the GTM and the proxy. The request is sent to the proxy and data is delivered

 

back from the proxy to the GTM (everything successful).

 

But the GTM marks the pool always as DOWN. Monitor state is "timeout" according to the gtm log.

 

From a network point of view, data delivery and proxy behaviour - everything's looking fine.

 

 

I tried a TCP and a HTTP monitor. Both doesn't work! Also with "\r\n\r\n" at the end

 

 

What's working, apart from that, is a simple TCP check if the proxy port (8080) is alive.

 

That's a simple HTTP monitor with a send string "GET /" to the service port.