dhtmlxAjax.getSync with non-xml-data

If the URL given in dhtmlxAjax.getSync returns non-xml-data, for example a json-string,
I receive that string but the firefox error console shows the error
“not well formed” or “syntax error”.
How can I fix this?

Make sure that you are not using xml content-type.

Ok, I’ve put

Context.Response.ContentType = "text/html";

to the webservice-code and then there are no error messages.