diff --git a/Web/app.py b/Web/app.py
index 99d11a3..cbbdf50 100755
--- a/Web/app.py
+++ b/Web/app.py
@@ -6853,11 +6853,9 @@ def register():
             password = request.form['password']
             name = (request.form.get('name') or '').strip()
             last_name = (request.form.get('last-name') or '').strip()
-            
-            # Generate username from name and last_name if not provided or empty
-            username = (request.form.get('username') or '').strip()
-            if not username:
-                username = us.build_username_from_name(name, last_name)
+
+            # Always generate username from abbreviation logic and auto-extend on collisions.
+            username = us.build_unique_username_from_name(name, last_name)
             
             permission_preset = (request.form.get('permission_preset') or 'standard_user').strip()
             use_custom_permissions = request.form.get('use_custom_permissions') == 'on'
@@ -6867,9 +6865,6 @@ def register():
             if not username or not password or not name or not last_name:
                 flash('Bitte füllen Sie alle Felder aus', 'error')
                 return redirect(url_for('register'))
-            if us.get_user(username):
-                flash('Benutzer existiert bereits', 'error')
-                return redirect(url_for('register'))
             if not us.check_password_strength(password):
                 flash('Passwort ist zu schwach', 'error')
                 return redirect(url_for('register'))
diff --git a/Web/templates/register.html b/Web/templates/register.html
index 61ae8c9..c382050 100755
--- a/Web/templates/register.html
+++ b/Web/templates/register.html
@@ -49,7 +49,7 @@
                         <span class="input-icon">👤</span>
                         <input type="text" id="username" name="username" placeholder="Automatisch aus Name und Nachname" readonly style="background-color: #f3f4f6; cursor: not-allowed;">
                     </div>
-                    <p class="anonymize-hint">Klarnamen werden nur zur Erzeugung des Benutzernamens und eines Kuerzels (z.B. SimFri) verwendet und nicht als Klarname gespeichert.</p>
+                    <p class="anonymize-hint">Klarnamen werden nur zur Erzeugung des Benutzernamens als Kürzel (z.B. SimFri) verwendet; bei Kollision wird automatisch ein Buchstabe mehr genommen.</p>
                 </div>
                 
                 <div class="form-group">
@@ -472,11 +472,11 @@ function generateUsername() {
     let username = '';
     
     if (firstName && lastName) {
-        username = firstName + lastName;
+        username = (firstName.slice(0, 3) + lastName.slice(0, 3));
     } else if (firstName) {
-        username = firstName;
+        username = firstName.slice(0, 6);
     } else if (lastName) {
-        username = lastName;
+        username = lastName.slice(0, 6);
     }
     
     // Set the username field
diff --git a/Web/user.py b/Web/user.py
index 649f112..770f313 100755
--- a/Web/user.py
+++ b/Web/user.py
@@ -59,9 +59,8 @@ def build_name_synonym(first_name, last_name=''):
 
 def build_username_from_name(first_name, last_name=''):
     """
-    Build a deterministic username from first and last name.
-    Format: firstnameLastname (CamelCase, e.g., 'SimonFrings' or 'SFrings')
-    Ensures uniqueness by allowing server to check existence.
+    Build a deterministic username abbreviation from first and last name.
+    Uses the same short alias logic (e.g. SimFri) and stores it lowercase.
     
     Args:
         first_name (str): First name
@@ -70,18 +69,38 @@ def build_username_from_name(first_name, last_name=''):
     Returns:
         str: Generated username
     """
+    alias = build_name_synonym(first_name, last_name)
+    return alias.lower()
+
+
+def build_unique_username_from_name(first_name, last_name=''):
+    """
+    Build a unique username based on the abbreviation logic.
+    If a collision occurs, increase the username by one additional letter
+    from the combined cleaned name until it is unique.
+    """
     first = _clean_name_fragment(first_name)
     last = _clean_name_fragment(last_name)
-    
-    if first and last:
-        # Full: FirstLast (e.g., 'SimonFrings')
-        return (first + last).lower()
-    elif first:
-        return first.lower()
-    elif last:
-        return last.lower()
-    else:
-        return 'user'
+    combined = (first + last).lower()
+
+    base_username = build_username_from_name(first_name, last_name)
+    if not combined:
+        combined = base_username or 'user'
+
+    start_len = len(base_username) if base_username else min(6, len(combined))
+    start_len = max(1, min(start_len, len(combined)))
+
+    # Main strategy: take one more letter on each collision.
+    for length in range(start_len, len(combined) + 1):
+        candidate = combined[:length]
+        if not get_user(candidate):
+            return candidate
+
+    # Fallback if full combined name is already taken repeatedly.
+    suffix = 2
+    while get_user(f"{combined}{suffix}"):
+        suffix += 1
+    return f"{combined}{suffix}"
 
 
 ACTION_PERMISSION_KEYS = (