try

cmd/server/main.go

@@ -1,9 +1,6 @@
 package main
 
 import (
-	"bytes"
-	"compress/flate"
-	"encoding/base64"
 	"encoding/json"
 	"fmt"
 	"html/template"
@@ -342,18 +339,9 @@ func buildDiagramsNetURL(r *http.Request, id string, xml string) string {
 		scheme = "https"
 	}
 	base := fmt.Sprintf("%s://%s/diagram?id=%s", scheme, r.Host, url.QueryEscape(id))
-	// For self-hosted instances that insist on proxying, avoid the remote fetch entirely by providing a compressed data
-	// URI in the fragment. Fragment data is not sent over the network, so this stays client-side and avoids proxy 404s.
-	target := base
-	if xml != "" {
-		var buf bytes.Buffer
-		w, _ := flate.NewWriter(&buf, flate.BestCompression)
-		_, _ = w.Write([]byte(xml))
-		_ = w.Close()
-		compressed := base64.StdEncoding.EncodeToString(buf.Bytes())
-		target = "data:application/octet-stream;base64," + compressed
-	}
-	return fmt.Sprintf("%s/?splash=0&ui=min&libs=aws4#U%s", drawioBaseURL, target)
+	encoded := url.QueryEscape(base)
+	// Ask draw.io not to proxy (`proxy=0`) so it fetches directly (CORS headers are already set on /diagram).
+	return fmt.Sprintf("%s/?splash=0&ui=min&libs=aws4&proxy=0&url=%s", drawioBaseURL, encoded)
 }
 
 func addCORS(w http.ResponseWriter) {