diff --git a/data/web/inc/functions.inc.php b/data/web/inc/functions.inc.php
index 1947ec465..49c443e11 100644
--- a/data/web/inc/functions.inc.php
+++ b/data/web/inc/functions.inc.php
@@ -3397,6 +3397,8 @@ function set_user_loggedin_session($user) {
   session_regenerate_id(true);
   $_SESSION['mailcow_cc_username'] = $user;
   $_SESSION['mailcow_cc_role'] = 'user';
+  // Update User-Agent after session regeneration to prevent validation errors
+  $_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
   $sogo_sso_pass = file_get_contents("/etc/sogo-sso/sogo-sso.pass");
   $_SESSION['sogo-sso-user-allowed'][] = $user;
   $_SESSION['sogo-sso-pass'] = $sogo_sso_pass;
diff --git a/data/web/inc/sessions.inc.php b/data/web/inc/sessions.inc.php
index 8f3192d70..7c44f1541 100644
--- a/data/web/inc/sessions.inc.php
+++ b/data/web/inc/sessions.inc.php
@@ -43,6 +43,9 @@ if (!isset($_SESSION['SESS_REMOTE_UA'])) {
 if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > $SESSION_LIFETIME)) {
   session_unset();
   session_destroy();
+  session_start();
+  // After destroying session, we need to reset the User-Agent for the new session
+  $_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
 }
 $_SESSION['LAST_ACTIVITY'] = time();
 
@@ -134,6 +137,12 @@ function session_check() {
     return true;
   }
   if (!isset($_SESSION['SESS_REMOTE_UA']) || ($_SESSION['SESS_REMOTE_UA'] != $_SERVER['HTTP_USER_AGENT'])) {
+    // In development mode, allow User-Agent changes (e.g., for responsive testing in dev tools)
+    // Validate UA is not empty and has reasonable length (most UAs are under 200 chars, 500 is safe upper limit)
+    if (isset($GLOBALS['DEV_MODE']) && $GLOBALS['DEV_MODE'] && !empty($_SERVER['HTTP_USER_AGENT']) && strlen($_SERVER['HTTP_USER_AGENT']) < 500) {
+      $_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
+      return true;
+    }
     $_SESSION['return'][] = array(
       'type' => 'warning',
       'msg' => 'session_ua'
diff --git a/data/web/inc/triggers.admin.inc.php b/data/web/inc/triggers.admin.inc.php
index df46a459c..4708b9e00 100644
--- a/data/web/inc/triggers.admin.inc.php
+++ b/data/web/inc/triggers.admin.inc.php
@@ -50,6 +50,8 @@ if (isset($_POST["login_user"]) && isset($_POST["pass_user"])) {
     session_regenerate_id(true);
 		$_SESSION['mailcow_cc_username'] = $login_user;
 		$_SESSION['mailcow_cc_role'] = "admin";
+		// Update User-Agent after session regeneration to prevent validation errors
+		$_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
 		header("Location: /admin/dashboard");
     die();
 	}
diff --git a/data/web/inc/triggers.domainadmin.inc.php b/data/web/inc/triggers.domainadmin.inc.php
index a9f913688..187b27320 100644
--- a/data/web/inc/triggers.domainadmin.inc.php
+++ b/data/web/inc/triggers.domainadmin.inc.php
@@ -7,6 +7,8 @@ if (!empty($_GET['sso_token'])) {
     session_regenerate_id(true);
     $_SESSION['mailcow_cc_username'] = $username;
     $_SESSION['mailcow_cc_role'] = 'domainadmin';
+    // Update User-Agent after session regeneration to prevent validation errors
+    $_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
     header('Location: /domainadmin/mailbox');
   }
 }
@@ -61,6 +63,8 @@ if (isset($_POST["login_user"]) && isset($_POST["pass_user"])) {
     session_regenerate_id(true);
 		$_SESSION['mailcow_cc_username'] = $login_user;
 		$_SESSION['mailcow_cc_role'] = "domainadmin";
+		// Update User-Agent after session regeneration to prevent validation errors
+		$_SESSION['SESS_REMOTE_UA'] = $_SERVER['HTTP_USER_AGENT'];
 		header("Location: /domainadmin/mailbox");
     die();
 	}