The inbox shortcode is blank for email assignees

This can happen on hosting services that cache aggressively (e.g. WP Engine). Make sure the page with the shortcode is not cached on the server. You may need to ask your host to remove the page from the cache.

If the page is not cached then there may be a security plugin blocking long URLs. see the link below.

Another possible reason for this is that the shortcode contains curly quotes instead of normal quotes. If you've copied and pasted the shortcode, open up the editor in text more and check the quotes inside the shortcode aren't curly quotes.