Are emojis fine ? Emojis are fetched from a CDN and may be blocked by a proxy or CSP rule.
If an image (like emoji) doesn't show up, right click > show image and copy its link here so we can see if somebody can access it.
Icons are a local Fontawesome copy and should not encounter these issues. Did you try multiple browsers ? Did you try clearing the cache (server+client) ?
Can you check you have an assets/fonts folder with files in it ? Flarum may not be installed properly.
