Hi all,
I found a solution in this forum. In Moodle 3.1.1 I added the highlighted line in mod/forum/mod_form.php
$options = array();
$options[FORUM_CHOOSESUBSCRIBE] = get_string('subscriptionoptional', 'forum');
$options[FORUM_FORCESUBSCRIBE] = get_string('subscriptionforced', 'forum');
$options[FORUM_INITIALSUBSCRIBE] = get_string('subscriptionauto', 'forum');
$options[FORUM_DISALLOWSUBSCRIBE] = get_string('subscriptiondisabled','forum');
$mform->addElement('select', 'forcesubscribe', get_string('subscriptionmode', 'forum'), $options);
$mform->addHelpButton('forcesubscribe', 'subscriptionmode', 'forum');
$mform->setDefault('forcesubscribe', '2');
Now the default setting for all forums at our site is set to "auto subscription."
With kind regards,
Hartmut