You have a JS error on home page which affects wpForo too. When you click on [add topic] button it doesn't open pop-up window to invite login/register. I think this is caused by the error. The error probably comes from a plugin with Champ name or so, probably that plugin is also affected by cache or optimizer plugins.
Ultimate addons for gutenberg.
If we do only what we can to do, we would never do anything.
@Robert So I was wrong and the Ultimate plugin can't do it.
TheChamp is a super socializer plugin. I disabled this plugin. Deleted cache. Ctrl + F5. I can't see a mistake in the console anymore. But when you click the add topic button, the login window doesn't appear.
This error only appears on the homepage. The rest of the forum works well.
If we do only what we can to do, we would never do anything.