Mingle is not compatible with the new block editor in WordPress v5 (a.k.a “Gutenberg”). You’ll need to install and activate the Classic Editor plugin to use Mingle with WordPress v5+. → https://en-gb.wordpress.org/plugins/classic-editor/
P.S. Your support licensed for Mingle has expired, that’s why you cannot access the Mingle forum. To get continued support for Mingle you will need to renew your support license for that theme.