It turned out to be a bug in the API page. If I send an ISO8601 formatted date that is not older than 72 hours, I do get a normal answer!!

One note: The date should be formatted according to iso8601, but the : must be encoded as %3A, not as a 4 digit unicode or utf8 encoding, not as a normal : which should be allowed according to http url encodings. For Swift that means that you cannot use the URLComponents, unfortunately.