SOAP service Call from non browser client with APM policy

We are configuring our F5 APM to authenticate SOAP service calls. We create a VS and add APM policy and when we access the service from any browser its works fine. Now my application team want to call the service form a .net client . When they call they are able to create a Proxy for the services but when try to consume the service they receive the below error :

" return The content type text/html; charset=utf-8 of the response message does not match the content type of the binding (text/xml; charset=utf-8). If using a custom encoder, be sure that the IsContentTypeSupported method is implemented properly. The first 1024 bytes of the response were: 'BIG-IP logout page

When we remove the APM policy from the VS everything works fine. the message says the response were HTML and the html codes seems the bigip logout page. I am not able to understand , why its returning a Big-IP logout page

" BIG-IP logout page "

the APM policy is simple Start ---> certificate check ----> Allow Fallback ----> Deny

Am I missing anything here ?

Any suggestion is highly appreciated.