You can edit file src/globals.inc.php located in idoit webroot directory

Find and comment string:
//$g_config["sess_time"]      = isys_settings::get('session.time', 300);

Temporary change it to:       
$g_config["sess_time"]      = 900;

After fixing timeout in GUI, you can delete temporary string and uncomment original

(Tested on i-doit v. 1.4.8 )