Hi Joeri,
OK, if the certificate is OK, please try the following to identify your HTTP call as soap call:
<call> ...<htta><par id="httpheader.SOAPAction" value="SOAPAction"><!--optional - you can specify here your own http header definitions - multiple elements allowed--></par><par id="htta.returnpltypeforce" value=""><!--optional--></par><par id="htta.returnpltypedefault" value=""><!--optional--></par></htta></call>
Best regards
Bastian