On a posting page, there is a link that allow you to see more smilies if those exist. However that link point to:

posting.php?mode=post&f=CURRENT_FORUM_NUMBER

which result in a blank page.

This bug can be seen here on phpbb.com to.

Fix:
Point that link to:

posting.php?mode=smilies

And when you are on it, place the same link on UCP -> Private messages -> Compose message page to. It's missing there.