diff --git a/static/js/components/resourceList.js b/static/js/components/resourceList.js
index 67f64d8e..a160ac3c 100644
--- a/static/js/components/resourceList.js
+++ b/static/js/components/resourceList.js
@@ -517,6 +517,7 @@ export class ResourceListComponent {
         const thumb = /** @type {HTMLImageElement | null} */ (el.querySelector('.file-thumb'));
         if (thumb) {
             thumb.addEventListener('error', () => {
+                console.log(`no thumbnail for ${file.id} (${file.name}), request thumbnail generation from client side`);
                 thumb.classList.add('hidden');
                 thumbnail?.queueGenerate(file, (dataUrl) => {
                     thumb.src = dataUrl;
diff --git a/static/js/features/thumbnail.js b/static/js/features/thumbnail.js
index 0d8c3a0d..17f23922 100644
--- a/static/js/features/thumbnail.js
+++ b/static/js/features/thumbnail.js
@@ -17,8 +17,11 @@ let _pdfjsLib = null;
  */
 async function getPdfjsLib() {
     if (_pdfjsLib) return _pdfjsLib;
-    // IMPORTANT: this hack (const lib=...) so tsc will not load vendors library
-    const lib = '../vendors/pdf.min.mjs';
+    // IMPORTANT: use an absolute path so the import resolves correctly both in
+    // dev mode (native ESM, module at /js/features/thumbnail.js) and in release
+    // mode (IIFE bundle at /js/app.{hash}.js — relative '../vendors/…' would
+    // incorrectly resolve to /vendors/… instead of /js/vendors/…).
+    const lib = '/js/vendors/pdf.min.mjs';
     _pdfjsLib = /** @type {any} */ (await import(lib));
     _pdfjsLib.GlobalWorkerOptions.workerSrc = '/js/vendors/pdf.worker.min.mjs';
     return _pdfjsLib;