the admin session had expired; after logout this warning came up
Please try this:
admin/index.php Line 501
if($mybb->input['action'] == "logout" && $mybb->user)
replace with
if(!empty($mybb->user['uid']) && $mybb->input['action'] == "logout")
has not been tested
thank you my friend; nice to see you here too
test is running, waiting... waiting... waiting...
edit: solved