HTTP Problems with an Asynchronous Proxy

I hope this is a simple question.

As a background - I am running PI 7.1 and ECC 5.0 (Basis 640).

I have created my outbound interfaces, set them up in SPROXY, and developed the calls that are feeding data to PI. I know that this part is working ok, as I can see the messages appear in SXI_MONITOR. However, when the message is in the queue, it will go to an automatic resend and then finally fail with the message:

Error when sending by HTTP (error code: 35, error text: HTTPIO_ERROR_CUSTOM_MYSAPSSO-Fehlermeldung

beim Senden der Daten.)

When I do a connection test in SPROXY, the second test (The HTTP connection of the R/3 application server must function correctly - this is service program SPROX_CHECK_HTTP_COMMUNICATION), I am being told that there is a communication error.

The long text of the error is not very helpful with:

Diagnosis

The HTTP communication of this application server is not functioning correctly.

When calling the method

IF_HTTP_CLIENT=>RECEIVE

the exception

HTTP_COMMUNICATION_FAILURE

was triggered.

The short text for this exception reads as follows:

Communication Error

Procedure

Contact your system administrator.

Resolving this HTTP problem is the first issue, with the second being the resolution of any additional proxy configuration. If there are any other steps that I should be looking at, please let me know.

Thanks in advance for any help you can provide,

Ben