tom23 I doubt PHP extensions play a role directly, as the Flarum embeds are a completely client-side feature, and the server-side formatter code is extremely basic without any special requirement. But it could be an incompatibility with another third-party extension that has missing requirements.
For checking PHP extensions, it depends on your hosting. If it's managed, there will likely be a page like 420forum's one in your admin panel. If it's a VPS you can check the PHP config files, I believe there's a CLI command, or one of the easiest options is to create a temporary PHP file calling phpinfo(); and opening it via the browser.
