Error sending test e-mail

I get this error when sending a test e-mail from wp plugin.
How may i fix the issue?

Response received
Response headers: Requests_Utility_CaseInsensitiveDictionary Object( [data:protected] => Array ( [date] => Wed, 12 Aug 2020 14:13:42 GMT [content-type] => application/json [content-length] => 181 [vary] => Accept [server] => msys-http [cache-control] => no-cache, no-store ))
Response body: { “errors”: [ { “message”: “invalid data format\/type”, “description”: “field ‘content.headers’ is of type ‘json_array’, but needs to be of type ‘json_object'”, “code”: “1300” } ] }
Error in transmission