By default, safe mode disables the frontend code of plugins, themes, and components. So if that fixes it, it is a problem with a customization. You can choose to disable only plugins or themes to narrow it down. Then you know for sure whether it’s a plugin or theme.
And then you can try to disable them one by one. Did you add any custom code?
The error I see in the browser console when looking at the forum did not help me to easily spot the customization causing the problem.
Discourse Theme Creator is a Discourse forum where you can test and showcase themes and theme components. Things happening there are not related to what happens in your forum.