diff --git a/background/style-manager.js b/background/style-manager.js
index 6542ebbd..d0d8cd17 100644
--- a/background/style-manager.js
+++ b/background/style-manager.js
@@ -253,6 +253,7 @@ const styleManager = (() => {
     if (style.id == null) {
       delete style.id;
     }
+    fixUsoMd5Issue(style);
     return db.exec('put', style)
       .then(event => {
         if (style.id == null) {
@@ -380,6 +381,7 @@ const styleManager = (() => {
         return;
       }
       for (const style of styleList) {
+        fixUsoMd5Issue(style);
         styles.set(style.id, {
           appliesTo: new Set(),
           data: style
@@ -479,4 +481,12 @@ const styleManager = (() => {
   function getUrlNoHash(url) {
     return url.split('#')[0];
   }
+
+  // The md5Url provided by USO includes a duplicate "update" subdomain (see #523),
+  // This fixes any already installed styles containing this error
+  function fixUsoMd5Issue(style) {
+    if (style && style.md5Url && style.md5Url.includes('update.update.userstyles')) {
+      style.md5Url = style.md5Url.replace('update.update.userstyles', 'update.userstyles');
+    }
+  }
 })();
diff --git a/background/style-via-api.js b/background/style-via-api.js
index e5fdf6ac..79f5c289 100644
--- a/background/style-via-api.js
+++ b/background/style-via-api.js
@@ -159,7 +159,7 @@ API_METHODS.styleViaAPI = !CHROME && (() => {
       return;
     }
     const tabFrames = cache.get(tabId);
-    if (frameId in tabFrames) {
+    if (tabFrames && frameId in tabFrames) {
       delete tabFrames[frameId];
       if (isEmpty(tabFrames)) {
         onTabRemoved(tabId);
diff --git a/content/install-hook-userstyles.js b/content/install-hook-userstyles.js
index 72ea6862..797d84ee 100644
--- a/content/install-hook-userstyles.js
+++ b/content/install-hook-userstyles.js
@@ -218,6 +218,14 @@
       });
   }
 
+  // USO providing md5Url as "https://update.update.userstyles.org/#####.md5"
+  // instead of "https://update.userstyles.org/#####.md5"
+  function tryFixMd5(style) {
+    if (style && style.md5Url && style.md5Url.includes('update.update')) {
+      style.md5Url = style.md5Url.replace('update.update', 'update');
+    }
+    return style;
+  }
 
   function getStyleJson() {
     return getResource(getStyleURL(), {responseType: 'json'})
@@ -236,6 +244,7 @@
             return style;
           });
       })
+      .then(tryFixMd5)
       .catch(() => null);
   }