The Register button doesn't show up for me when I'm logged in. Do you have a custom navbar?
Is this what you have?
HTML Code:
<if condition="$show['registerbutton']">
<a href="$vboptions[bburl]/register.php$session[sessionurl_q]" rel="nofollow">$vbphrase[register]</a>
</if>
Perhaps change it to:
HTML Code:
<if condition="$show['guest']">
<a href="$vboptions[bburl]/register.php$session[sessionurl_q]" rel="nofollow">$vbphrase[register]</a>
</if>
(I have not tested this.)