I found the xhtml bug for the missing selfclosing tag:

in /ubbthreads/ubbt.inc.php line 806:
Code
$Body = preg_replace("/(\[IMG\]|\[{$ubbt_lang['TEXT_IMAGE']}\])http([^\[]*)\.(gif|jpg|png)(\[\/IMG\]|\[\/{$ubbt_lang['TEXT_IMAGE']}\])/i","<img src=\"http\\2.\\3\">",$Body);

should be:
Code
$Body = preg_replace("/(\[IMG\]|\[{$ubbt_lang['TEXT_IMAGE']}\])http([^\[]*)\.(gif|jpg|png)(\[\/IMG\]|\[\/{$ubbt_lang['TEXT_IMAGE']}\])/i","<img src=\"http\\2.\\3\" alt=\"\" />",$Body);

my board: http://www.dragonclan-forum.de
my hobby: http://www.biker-reise.de
Ich kann bei Fragen zu UBBthreads in Deutsch weiterhelfen oder es zumindest versuchen