JSON received from the API:

{
    "service-error": {
        "status": {
            "statusCode": "AUTHENTICATION_ERROR",
            "statusText": "Invalid API Key"
        }
    }
}

API-Server does answer, but with an error-message:
401 Unauthorized (RFC 7235) - Check what the API expects!
n opensearch:totalResults = u003cbru003en opensearch:startIndex = u003cbru003en opensearch:itemsPerPage = u003cbru003en n @role = {search-results.opensearch:Query.@role}u003cbru003en @searchTerms = {search-results.opensearch:Query.@searchTerms}u003cbru003en @startPage = {search-results.opensearch:Query.@startPage}u003cbru003en n link:u003cbru003en u003cbru003en @_fa = {search-results.link.@_fa}u003cbru003en @ref = {search-results.link.@ref}u003cbru003en @href = {search-results.link.@href}u003cbru003en @type = {search-results.link.@type}u003cbru003en u003cbru003en entry:u003cbru003en u003cbru003en @_fa = {search-results.entry.@_fa}u003cbru003en link:u003cbru003en u003cbru003en @_fa = {search-results.entry.link.@_fa}u003cbru003en @ref = {search-results.entry.link.@ref}u003cbru003en @href = {search-results.entry.link.@href}u003cbru003en u003cbru003en prism:url = u003cbru003en dc:identifier = u003cbru003en eid = u003cbru003en dc:title = u003cbru003en dc:creator = u003cbru003en prism:publicationName = u003cbru003en prism:issn = u003cbru003en prism:eIssn = u003cbru003en prism:volume = u003cbru003en prism:issueIdentifier = u003cbru003en prism:pageRange = u003cbru003en prism:coverDate = u003cbru003en prism:coverDisplayDate = u003cbru003en prism:doi = u003cbru003en citedby-count = u003cbru003en affiliation:u003cbru003en u003cbru003en @_fa = {search-results.entry.affiliation.@_fa}u003cbru003en affilname = u003cbru003en affiliation-city = u003cbru003en affiliation-country = u003cbru003en u003cbru003en pubmed-id = u003cbru003en prism:aggregationType = u003cbru003en subtype = u003cbru003en subtypeDescription = u003cbru003en article-number = u003cbru003en source-id = u003cbru003en openaccess = u003cbru003en openaccessFlag = u003cbru003en n value:u003cbru003en u003cbru003en $ = u003cbru003en u003cbru003en n n value:u003cbru003en u003cbru003en $ = u003cbru003en u003cbru003en n prism:isbn:u003cbru003en u003cbru003en @_fa = {search-results.entry.prism:isbn.@_fa}u003cbru003en $ = u003cbru003en u003cbru003en pii = u003cbru003en u003cbru003en