resolve issues with new features

frontend/src/components/admin/UserOverviewTab.tsx

@@ -40,7 +40,7 @@ function UserOverviewTab() {
   const [sortKey, setSortKey] = useState<SortKey>('name');
   const [sortDir, setSortDir] = useState<SortDir>('asc');
 
-  const { data: users, isLoading } = useQuery({
+  const { data: users, isLoading, isError } = useQuery({
     queryKey: ['admin', 'users'],
     queryFn: adminApi.getUsers,
   });
@@ -82,6 +82,17 @@ function UserOverviewTab() {
     return <Box sx={{ display: 'flex', justifyContent: 'center', p: 4 }}><CircularProgress /></Box>;
   }
 
+  if (isError) {
+    return (
+      <Box sx={{ p: 4, textAlign: 'center' }}>
+        <Typography color="error" gutterBottom>Benutzerdaten konnten nicht geladen werden.</Typography>
+        <Typography variant="body2" color="text.secondary">
+          Bitte versuchen Sie es später erneut.
+        </Typography>
+      </Box>
+    );
+  }
+
   return (
     <Box>
       <Box sx={{ display: 'flex', justifyContent: 'space-between', alignItems: 'center', mb: 2 }}>