scottklement.com

I have a problem with the HTTPAPI; Diacritics and ligatures e.g.(äëç and æß) seems not be supported by the Api.
The moment it encounter the first result-field with a character in this category,
the program stops with error: "XML parse failed at line 1, col 319: not well-formed (invalid token)"

Not true, it fully supports all unicode characters (though, if they don't exist in your flavor of EBCDIC, you might lose them when eventually loading them into RPG variables -- but, that is not what is happening in this case, it is failing when trying to interpret the XML.)

Most likely, you have your HTTPAPI configured with the wrong CCSIDs.