diff --git a/diagnostic/server.js b/diagnostic/server.js
index bdde67f..d5662b9 100644
--- a/diagnostic/server.js
+++ b/diagnostic/server.js
@@ -149,7 +149,9 @@ async function connectGateway() {
       state.gateway.handshakeOk = true;
       state.gateway.lastError = null;
     } else {
-      const error = response.error || JSON.stringify(response).slice(0, 200);
+      const error = typeof response.error === "string"
+        ? response.error
+        : JSON.stringify(response.error || response).slice(0, 300);
       throw new Error(`Handshake fehlgeschlagen: ${error}`);
     }