diff -urN ../roundcubemail-1.6.1.orig/plugins/markasjunk/drivers/dir_learn.php ./plugins/markasjunk/drivers/dir_learn.php
--- ../roundcubemail-1.6.1.orig/plugins/markasjunk/drivers/dir_learn.php	2023-01-23 22:03:13.000000000 +0200
+++ ./plugins/markasjunk/drivers/dir_learn.php	2023-04-13 18:13:06.230324000 +0300
@@ -47,6 +47,7 @@
 
         $debug    = $rcube->config->get('markasjunk_debug');
         $filename = $rcube->config->get('markasjunk_filename');
+        if ($filename === null) $filename = '';
         $filename = str_replace('%u', $_SESSION['username'], $filename);
         $filename = str_replace('%t', ($spam) ? 'spam' : 'ham', $filename);
         $filename = str_replace('%l', $rcube->user->get_username('local'), $filename);