diff --git a/background/storage.js b/background/storage.js
index b57cdf62..55c47590 100644
--- a/background/storage.js
+++ b/background/storage.js
@@ -174,6 +174,7 @@ function getStyles(options) {
 
   return dbExec('getAll').then(event => {
     cachedStyles.list = event.target.result || [];
+    cachedStyles.list.forEach(fixUsoMd5Issue);
     cachedStyles.byId.clear();
     for (const style of cachedStyles.list) {
       cachedStyles.byId.set(style.id, style);
@@ -264,6 +265,14 @@ function filterStyles({
 }
 
 
+// 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');
+  }
+}
+
 function filterStylesInternal({
   // js engines don't like big functions (V8 often deoptimized the original filterStyles)
   // it also makes sense to extract the less frequently executed code
@@ -350,6 +359,8 @@ function saveStyle(style) {
   let existed;
   let codeIsUpdated;
 
+  fixUsoMd5Issue(style);
+
   return maybeCalcDigest()
     .then(maybeImportFix)
     .then(decide);
diff --git a/content/install-hook-userstyles.js b/content/install-hook-userstyles.js
index d26a88af..7ebdbd22 100644
--- a/content/install-hook-userstyles.js
+++ b/content/install-hook-userstyles.js
@@ -238,6 +238,14 @@
     });
   }
 
+  // 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'})
@@ -256,6 +264,7 @@
           });
         }));
       })
+      .then(tryFixMd5)
       .catch(() => null);
   }