I have the same problem like you. Is strange because this is 3th server and second time when this problem occur. So, i have 2 server working fine. Same settings, no problem. On the second server this problem appeared but i says: the problem is me because i want to install roundcube 1.4 instead 1.3 like in tutorial. So I reinstalled entire server, step by step and everything was fine. Now the problem reappeared and I don't see any problem in apache log. I don't know where is the point to start debug. I don't want to reinstall entire server EDITED!!! PROBLEM SOLVED: It was my mistake. In /opt/roundcube/plugins/ispconfig3_account/config/config.inc.php It was write http instead https. In my case https is correct mode. $rcmail_config['soap_url'] = 'https://myserver: port/remote/' <?php $rcmail_config['identity_limit'] = false; $rcmail_config['remote_soap_user'] = 'remote user'; $rcmail_config['remote_soap_pass'] = 'remote user password'; $rcmail_config['soap_url'] = 'https://myserver : port/remote/'; ?> Thank You Till for your tutorials!