diff -ruN roundcube.orig/index.php roundcube/index.php
--- roundcube.orig/index.php	2021-04-26 14:48:33.169655413 +0200
+++ roundcube/index.php	2021-04-26 14:54:07.594479133 +0200
@@ -210,6 +210,10 @@
     if (!$RCMAIL->session->check_auth()) {
         $RCMAIL->kill_session();
         $session_error = 'sessionerror';
+        if (!empty($_SERVER['PHP_AUTH_USER'])) {
+            header('Location: https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
+            exit;
+        }
     }
 }