So far we haven’t implemented the upgrade jet but there is a temporary solution which helps for the time being.
Instead of using the "Call web service" action it is also possible to use the "Web request" action with which we can get the whole XML result.
When having the XLM result stored in a multiline variable I used the "Query XML" action to get the byte string.
Query XML allows to edit the XPath where the wrongly generated XPath
can be modified to