I’m new here so I may be mistaken, but I think I was just having the same issue with formatting.
In your body it you have quotes around the data types i.e {“text”:“test1”} should be {text:“test1”} and so on for each data type. Be sure to check the data type each pair needs, I believe numbers7 would need an int not a string.
String body = “{ query: mutation { change_multiple_column_values(board_id: XXXXXXXX, item_id: XXXXXXXX, column_values: {text:“test1”, numbers7:0, date: { date : “1955-10-08” }}) { id }}}";