That's in the
STANDARD_ERROR template.
Look for this code right before the </form> line:
Code:
<div class="smallfont"><phrase 1="register.php?$session[sessionurl]do=signup">$vbphrase[admin_required_register]</phrase></div>
If it's not there, add it back.
If it is there then check your phrase manager, the phrase
admin_required_register is probably blank or missing.