Thanks for your test. My installation was a clean, fresh installation. I made changes to the config file.
$rcmail_config = array();
$rcmail_config['debug_level'] = 1;
$rcmail_config['enable_caching'] = TRUE;
$rcmail_config['message_cache_lifetime'] = '10d';
$rcmail_config['auto_create_user'] = TRUE;
$rcmail_config['default_host'] = 'ssl://XXXX.webpack.hosteurope.de';
$rcmail_config['default_port'] = 993;
$rcmail_config['imap_auth_type'] = null;
$rcmail_config['username_domain'] = '';
$rcmail_config['mail_domain'] = '';
$rcmail_config['virtuser_file'] = '';
$rcmail_config['virtuser_query'] = '';
$rcmail_config['smtp_server'] = '';
$rcmail_config['smtp_port'] = 25;
$rcmail_config['smtp_user'] = '%u';
$rcmail_config['smtp_pass'] = '%p';
$rcmail_config['smtp_auth_type'] = '';
$rcmail_config['smtp_helo_host'] = '';
$rcmail_config['smtp_log'] = TRUE;
$rcmail_config['list_cols'] = array('from', 'date', 'subject', 'size');
$rcmail_config['skin_path'] = 'skins/default/';
$rcmail_config['skin_include_php'] = FALSE;
$rcmail_config['temp_dir'] = 'temp/';
$rcmail_config['log_dir'] = 'logs/';
$rcmail_config['session_lifetime'] = 10;
$rcmail_config['ip_check'] = false;
$rcmail_config['double_auth'] = false;
$rcmail_config['des_key'] = 'rcmail-!24ByteDESkey*Str';
$rcmail_config['language'] = 'de';
$rcmail_config['date_short'] = 'D H:i';
$rcmail_config['date_long'] = 'd.m.Y H:i';
$rcmail_config['date_today'] = 'H:i';
$rcmail_config['useragent'] = 'Webmailer RC';
$rcmail_config['product_name'] = 'Webmail';
$rcmail_config['imap_root'] = '';
$rcmail_config['drafts_mbox'] = 'Drafts';
$rcmail_config['junk_mbox'] = 'Junk';
$rcmail_config['sent_mbox'] = 'Sent';
$rcmail_config['trash_mbox'] = 'Trash';
$rcmail_config['default_imap_folders'] = array('INBOX', 'Drafts', 'Sent', 'Junk', 'Trash');
$rcmail_config['create_default_folders'] = TRUE;
$rcmail_config['protect_default_folders'] = TRUE;
$rcmail_config['skip_deleted'] = FALSE;
$rcmail_config['read_when_deleted'] = TRUE;
$rcmail_config['flag_for_deletion'] = TRUE;
$rcmail_config['mdn_requests'] = 0;
$rcmail_config['default_charset'] = 'ISO-8859-1';
$rcmail_config['enable_spellcheck'] = TRUE;
$rcmail_config['spellcheck_uri'] = '';
$rcmail_config['spellcheck_languages'] = NULL;
$rcmail_config['generic_message_footer'] = '';
$rcmail_config['http_received_header'] = TRUE;
$rcmail_config['mail_header_delimiter'] = NULL;
$rcmail_config['session_domain'] = '';
$rcmail_config['address_book_type'] = 'sql';
$rcmail_config['dont_override'] = array();
$rcmail_config['include_host_config'] = false;
$rcmail_config['max_pagesize'] = 200;
$rcmail_config['mime_magic'] = '/usr/share/misc/magic';
$rcmail_config['message_sort_col'] = 'date';
$rcmail_config['message_sort_order'] = 'DESC';
$rcmail_config['enable_installer'] = false;
$rcmail_config['pagesize'] = 40;
$rcmail_config['timezone'] = intval(date('O'))/100 - date('I');
$rcmail_config['dst_active'] = (bool)date('I');
$rcmail_config['prefer_html'] = TRUE;
$rcmail_config['htmleditor'] = FALSE;
$rcmail_config['prettydate'] = TRUE;
$rcmail_config['draft_autosave'] = 300;
$rcmail_config['preview_pane'] = TRUE;
$rcmail_config['logout_purge'] = FALSE;
$rcmail_config['logout_expunge'] = FALSE;
$rcmail_config['delete_always'] = false;
$rcmail_config['log_logins'] = false;
Thanks in advance,
majobu