Your forums use a lot of customizations. So my first step would always be to rule out that it’s one of them. Does it also happen when you use https://dessein-tech.com/tag/au2026/152?ascending=false&order=created&safe_mode=no_themes,no_unofficial_plugins?
Safe mode that disables plugins is going to help because it disables the chat plugin.
If safe mode disabling themes helps, my first guess would be Chat Sidebar, where the description mentions