$ZMI.get_rest_api_url(href) returns wrong URL on Home in a ZMS Production with domain

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I want to add a hyperlink in a ZMSTextarea on the homepage https://zmsseminar.uni-koeln.de/. After I mark a text and click 'Add Link', the ZMIObjectTree opens and makes an AJAX request to https://++rest_api/zmsseminar.uni-koeln.de/get_parent_nodes?lang=ger, which returns the JavaScript error net::ERR_NAME_NOT_RESOLVED