diff --git a/src/UI/UIWindowProgress.js b/src/UI/UIWindowProgress.js
index 85dc5da9..3f111858 100644
--- a/src/UI/UIWindowProgress.js
+++ b/src/UI/UIWindowProgress.js
@@ -26,9 +26,8 @@ import Button from './Components/Button.js';
  * @param operation_id If provided, is saved in the data-operation-id attribute, for later lookup.
  * @param show_progress Enable a progress bar, and display `(foo%)` after the status message
  * @param on_cancel A callback run when the Cancel button is clicked. Without it, no Cancel button will appear.
- * @returns {Promise<{set_progress: *, set_status: *, close: *, element: Element}>} Object for managing the progress dialog
+ * @returns {Promise<{set_progress: *, set_status: *, close: *, show_error: *, element: Element}>} Object for managing the progress dialog
  * @constructor
- * TODO: Error display
  * TODO: Debouncing logic (show only after a delay, then hide only after a delay)
  */
 async function UIWindowProgress({
@@ -37,30 +36,47 @@ async function UIWindowProgress({
     on_cancel = null,
 } = {}){
     const placeholder_cancel_btn = Placeholder();
+    const placeholder_ok_btn = Placeholder();
 
     let h = '';
     h += `<div ${operation_id ? `data-operation-id="${operation_id}"` : ''}>`;
-        h += `<div style="display: flex; align-items: center;">`;
-            // spinner
-            h += `<svg style="margin-right: 7px; overflow: visible;" xmlns="http://www.w3.org/2000/svg" height="24" width="24" viewBox="0 0 24 24"><title>circle anim</title><g fill="#212121" class="nc-icon-wrapper"><g class="nc-loop-circle-24-icon-f"><path d="M12 24a12 12 0 1 1 12-12 12.013 12.013 0 0 1-12 12zm0-22a10 10 0 1 0 10 10A10.011 10.011 0 0 0 12 2z" fill="#212121" opacity=".4"></path><path d="M24 12h-2A10.011 10.011 0 0 0 12 2V0a12.013 12.013 0 0 1 12 12z" data-color="color-2"></path></g><style>.nc-loop-circle-24-icon-f{--animation-duration:0.5s;transform-origin:12px 12px;animation:nc-loop-circle-anim var(--animation-duration) infinite linear}@keyframes nc-loop-circle-anim{0%{transform:rotate(0)}100%{transform:rotate(360deg)}}</style></g></svg>`;
-            // Progress report
-            h += `<div style="font-size:15px; overflow: hidden; flex-grow: 1; text-overflow: ellipsis; white-space: nowrap;">
-                <span class="progress-msg">${i18n('preparing')}</span>`;
+        h += `<div class="progress-running">`;
+            h += `<div style="display: flex; align-items: center; gap: 7px;">`;
+                // spinner
+                h += `<svg style="overflow: visible;" xmlns="http://www.w3.org/2000/svg" height="24" width="24" viewBox="0 0 24 24"><title>circle anim</title><g fill="#212121" class="nc-icon-wrapper"><g class="nc-loop-circle-24-icon-f"><path d="M12 24a12 12 0 1 1 12-12 12.013 12.013 0 0 1-12 12zm0-22a10 10 0 1 0 10 10A10.011 10.011 0 0 0 12 2z" fill="#212121" opacity=".4"></path><path d="M24 12h-2A10.011 10.011 0 0 0 12 2V0a12.013 12.013 0 0 1 12 12z" data-color="color-2"></path></g><style>.nc-loop-circle-24-icon-f{--animation-duration:0.5s;transform-origin:12px 12px;animation:nc-loop-circle-anim var(--animation-duration) infinite linear}@keyframes nc-loop-circle-anim{0%{transform:rotate(0)}100%{transform:rotate(360deg)}}</style></g></svg>`;
+                // Progress report
+                h += `<div style="font-size:15px; overflow: hidden; flex-grow: 1; text-overflow: ellipsis; white-space: nowrap;">
+                    <span class="progress-msg">${i18n('preparing')}</span>`;
+                if (show_progress) {
+                    h += ` (<span class="progress-percent">0%</span>)`;
+                }
+                h += `</div>`;
+            h +=`</div>`;
             if (show_progress) {
-                h += ` (<span class="progress-percent">0%</span>)`;
+                h += `<div class="progress-bar-container" style="margin-top:20px;">`;
+                    h += `<div class="progress-bar"></div>`;
+                h += `</div>`;
             }
+            if (on_cancel) {
+                h += `<div style="display: flex; justify-content: flex-end;">`;
+                    h += placeholder_cancel_btn.html;
+                h += `</div>`;
+            }
+        h += `</div>`;
+        h += `<div class="progress-error" style="display: none">`;
+            h += `<div style="display: flex; align-items: center; gap: 7px;">`;
+                // Alert icon
+                h += `<img style="width:24px; height:24px;" src="${html_encode(window.icons['warning-sign.svg'])}" />`;
+                // Progress report
+                h += `<div style="font-size:15px; overflow: hidden; flex-grow: 1; text-overflow: ellipsis; white-space: nowrap;">
+                    <span class="progress-error-title"></span>`;
+                h += `</div>`;
             h += `</div>`;
-        h +=`</div>`;
-        if (show_progress) {
-            h += `<div class="progress-bar-container" style="margin-top:20px;">`;
-                h += `<div class="progress-bar"></div>`;
-            h += `</div>`;
-        }
-        if (on_cancel) {
+            h += `<p class="progress-error-message"></p>`;
             h += `<div style="display: flex; justify-content: flex-end;">`;
-                h += placeholder_cancel_btn.html;
+                h += placeholder_ok_btn.html;
             h += `</div>`;
-        }
+        h += `</div>`;
     h += `</div>`;
 
     const el_window = await UIWindow({
@@ -106,6 +122,15 @@ async function UIWindowProgress({
         cancel_btn.attach(placeholder_cancel_btn);
     }
 
+    const ok_btn = new Button({
+        label: i18n('ok'),
+        style: 'small',
+        on_click: () => {
+            $(el_window).close();
+        },
+    });
+    ok_btn.attach(placeholder_ok_btn);
+
     return {
         element: el_window,
         set_status: (text) => {
@@ -118,7 +143,12 @@ async function UIWindowProgress({
         close: () => {
             $(el_window).close();
         },
-        // TODO: show_error(), which replaces the window content with a title, error message, and OK button
+        show_error: (title, message) => {
+            el_window.querySelector('.progress-running').style.display = 'none';
+            el_window.querySelector('.progress-error').style.display = 'block';
+            el_window.querySelector('.progress-error-title').innerText = title;
+            el_window.querySelector('.progress-error-message').innerText = message;
+        },
     };
 }
 
diff --git a/src/helpers.js b/src/helpers.js
index 77c8abe3..f4e3d134 100644
--- a/src/helpers.js
+++ b/src/helpers.js
@@ -2771,9 +2771,7 @@ window.upload_items = async function(items, dest_path){
             },
             // error
             error: async function(err){
-                // TODO: Display error in progress dialog
-                upload_progress_window.close();
-                // UIAlert(err?.message ?? 'An error occurred while uploading.');
+                upload_progress_window.show_error(i18n('error_uploading_files'), err.message);
                 // remove from active_uploads
                 delete window.active_uploads[opid];
             },
diff --git a/src/i18n/translations/en.js b/src/i18n/translations/en.js
index 14ca930f..ad2abc47 100644
--- a/src/i18n/translations/en.js
+++ b/src/i18n/translations/en.js
@@ -114,6 +114,7 @@ const en = {
         enlarged_qr_code: "Enlarged QR Code",
         enter_password_to_confirm_delete_user: "Enter your password to confirm account deletion",
         error_unknown_cause: "An unknown error occurred.",
+        error_uploading_files: "Failed to upload files",
         feedback: "Feedback",
         feedback_c2a: "Please use the form below to send us your feedback, comments, and bug reports.",
         feedback_sent_confirmation: "Thank you for contacting us. If you have an email associated with your account, you will hear back from us as soon as possible.",