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 )