If you're feeling adventurous you can fix it your self. Open up core/inc/bigtree/admin.php in your favorite text editor and look for the createTag() function. The issue is that the object $cms is out of scope and needs to be globalized:
// Change:
function createTag($tag) {
$tag = strtolower(html_entity_decode($tag));
// To:
function createTag($tag) {
global $cms;
$tag = strtolower(html_entity_decode($tag));
The great wisdom of the docs has yielded no answers. What setting have I forgotten, oh people of the Big Tree?
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