... just for debugging i wrote the ajax HTML into a text file. I found that my mistake was a not close comment.

The XML page cannot be displayed
Cannot view XML input using XSL style sheet. Please correct the error and then click the Refresh button, or try again later.

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A comment was not closed. Error processing resource 'file:///C:/Daten/xml_reply_action_tilg.xml'. Line 10, Position 5