This happened to us in WP 5.6. We had to downgrade to 5.4 to get it to work again. I think it's that Muut isn't updating anymore, so well.. it looks like this might be dead software.
You can embed the forum in your WP site.
We had the same problem with the WP upgrade. The fix is to install the WP plugin "Enable jQuery Migrate Helper" and use this to downgrade the version of jQuery used on your WP site.