diff --git a/public/calibration.html b/public/calibration.html
index 8f62262..e79abb4 100644
--- a/public/calibration.html
+++ b/public/calibration.html
@@ -451,6 +451,13 @@
       }
     });
 
+    // Hilfsfunktion: fetch-Response sicher als Text lesen und in lesbaren Fehler umwandeln
+    async function safeJson(r) {
+      const raw = await r.text().catch(() => '');
+      try   { return { ok: r.ok, status: r.status, data: JSON.parse(raw) }; }
+      catch { return { ok: r.ok, status: r.status, data: null, raw: raw.slice(0, 300) }; }
+    }
+
     // "NPZ speichern" → an Webcam-Service übertragen
     document.getElementById('btn-upload-npz').addEventListener('click', async () => {
       const camera = document.getElementById('cam-select-calib').value;
@@ -458,15 +465,17 @@
 
       logC(`NPZ wird an Webcam-Service übertragen (${camera}) …`);
       try {
-        const r = await fetch('/api/calibration/upload-npz', {
+        const { ok, status, data, raw } = await safeJson(await fetch('/api/calibration/upload-npz', {
           method:  'POST',
           headers: { 'Content-Type': 'application/json' },
           body:    JSON.stringify({ camera }),
-        });
-        const d = await r.json();
-        if (d.error) { logC(`❌ Fehler: ${d.error}`); return; }
-        logC(`✅ Gespeichert: ${d.webcam?.saved}  (${d.size} Bytes)`);
-        logC(`   calibrationUrl: ${d.webcam?.calibrationUrl}`);
+        }));
+        if (!ok || data?.error) {
+          logC(`❌ HTTP ${status}: ${data?.error ?? raw ?? '(kein Fehlertext)'}`);
+          return;
+        }
+        logC(`✅ Gespeichert: ${data.webcam?.saved}  (${data.size} Bytes)`);
+        logC(`   calibrationUrl: ${data.webcam?.calibrationUrl}`);
       } catch (err) {
         logC(`❌ Fehler: ${err}`);
       }
@@ -489,8 +498,11 @@
         });
 
         if (!response.ok) {
-          const err = await response.json().catch(() => ({ error: response.statusText }));
-          logC(`Fehler: ${err.error}`);
+          const raw = await response.text().catch(() => '');
+          let msg;
+          try   { msg = JSON.parse(raw).error || raw; }
+          catch { msg = raw.slice(0, 300) || response.statusText; }
+          logC(`❌ HTTP ${response.status}: ${msg || '(kein Fehlertext – evtl. Server neu starten?)'}`);
           return;
         }