backport of commit 2d1c3db (#27951)

Co-authored-by: miagilepner <[email protected]>

changelog/27939.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+core (enterprise): Fix 500 errors that occurred querying `sys/internal/ui/mounts` for a mount prefixed by a namespace path when path filters are configured.
+```

vault/logical_system.go

@@ -5074,6 +5074,14 @@ func (b *SystemBackend) pathInternalUIMountRead(ctx context.Context, req *logica
 		routerPrefix = credentialRoutePrefix
 	}
 
+	// the mount's namespace is (at least partially) in the request path and not
+	// in the request's context, so we need to add the namespace from the
+	// request path to the router prefix
+	if me.NamespaceID != ns.ID {
+		namespaceRouterPrefix := strings.TrimPrefix(me.Namespace().Path, ns.Path)
+		routerPrefix = namespaceRouterPrefix + routerPrefix
+	}
+
 	filtered, err := b.Core.checkReplicatedFiltering(ctx, me, routerPrefix)
 	if err != nil {
 		return nil, err