Clj-http exception: ExceptionInfo clj-http: status 415 clj-http.client/wrap-exceptions/fn--1863 (client.clj:196)

The default for clj-http is to throw exceptions on >=400 status codes. So once you see an exception, dig deeper with e.g. (pst) or how you usually deal with the exception. E.g.

(c/get "https://httpbin.org/status/400")
; Execution error (ExceptionInfo) at slingshot.support/stack-trace (support.clj:201).
; clj-http: status 400
(pst)
; ...
; clojure.lang.ExceptionInfo: clj-http: status 400
;                      body: ""
;                    cached: nil
;                  chunked?: false
;                   headers: {"Date" "Tue, 04 Feb 2020 19:37:14 GMT",
;                             "Content-Type" "text/html; charset=utf-8",
;                             "Content-Length" "0",
;                             "Connection" "close",
;                             "Server" "gunicorn/19.9.0",
;                             "Access-Control-Allow-Origin" "*",
;                             "Access-Control-Allow-Credentials" "true"}
;               http-client: #object[org.apache.http.impl.client.InternalHttpClient 0x3cc148d "org.apache.http.impl.client.InternalHttpClient@3cc148d"]
;                    length: 0
;     orig-content-encoding: nil
;          protocol-version: {:name "HTTP", :major 1, :minor 1}
;             reason-phrase: "BAD REQUEST"
;               repeatable?: false
;              request-time: 429
;                    status: 400
;                streaming?: false
;           trace-redirects: []
;                      type: :clj-http.client/unexceptional-status
; ...

If you don't want this feature, you can disable it with the option :throw-exceptions false:

(c/get "https://httpbin.org/status/400" {:throw-exceptions false})
; {:body "",
;  :cached nil,
;  :chunked? false,
;  :headers {"Access-Control-Allow-Credentials" "true",
;            "Access-Control-Allow-Origin" "*",
;            "Connection" "close",
;            "Content-Length" "0",
;            "Content-Type" "text/html; charset=utf-8",
;            "Date" "Tue, 04 Feb 2020 19:40:20 GMT",
;            "Server" "gunicorn/19.9.0"},
;  :http-client #<org.apache.http.impl.client.InternalHttpClient@27bde886>,
;  :length 0,
;  :orig-content-encoding nil,
;  :protocol-version {:major 1, :minor 1, :name "HTTP"},
;  :reason-phrase "BAD REQUEST",
;  :repeatable? false,
;  :request-time 432,
;  :status 400,
;  :streaming? false,
;  :trace-redirects []}

Or if you really want to see what is going on, you can also use :debug true to let the underlying http librarly log "everything":

(c/get "https://httpbin.org/status/400" {:debug true :throw-exceptions false})
; Request: nil
; {:user-info nil,
;  :use-header-maps-in-response? true,
;  :body-type nil,
;  :debug true,
;  :headers {"accept-encoding" "gzip, deflate"},
;  :server-port nil,
;  :url "https://httpbin.org/status/400",
;  :flatten-nested-keys (:query-params),
;  :throw-exceptions false,
;  :uri "/status/400",
;  :server-name "httpbin.org",
;  :query-string nil,
;  :body nil,
;  :scheme :https,
;  :request-method :get}
; HttpRequest:
; {:config nil,
;  :method "GET",
;  :requestLine
;  #object[org.apache.http.message.BasicRequestLine 0x648100de "GET https://httpbin.org/status/400 HTTP/1.1"],
;  :aborted false,
;  :params
;  #object[org.apache.http.params.BasicHttpParams 0x3d1319b "org.apache.http.params.BasicHttpParams@3d1319b"],
;  :protocolVersion
;  #object[org.apache.http.HttpVersion 0x34b63def "HTTP/1.1"],
;  :URI
;  #object[java.net.URI 0x51ba8929 "https://httpbin.org/status/400"],
;  :class org.apache.http.client.methods.HttpGet,
;  :allHeaders
;  [#object[org.apache.http.message.BasicHeader 0x39a8905b "Connection: close"],
;   #object[org.apache.http.message.BasicHeader 0x6d6c4775 "accept-encoding: gzip, deflate"]]}
; {:body "",
;  :cached nil,
;  :chunked? false,
; ...

The HTTP client library appears to be reporting that the server rejected your request with HTTP status code 415. The Internet Engineering Task Force defined HTTP status code 415 in RFC 7231.