This sounds like the special characters are causing the web service call to view the data as malformed. When this happens I like to encode the string first. This may cause some issues at the other end but at least the data will get there. I use a static method to do this. On the Toolbox select Choose Items, then select the Pega Robotics Static Methods tab. Select the highlighted items in the screenshot below to add the HtmlDecode and HtmlEncode methods to your toolbox.
No to use this, just encode your Body before adding to the case.