1) There is a small typo in template postlist.tpl:
<a style="text-decoration: none;" href="{$config.BASE_URL}/ubbthreads.php?ubb=newpost&Board={$Board}" rel=\"nofollow\">
should be:
<a style="text-decoration: none;" href="{$config.BASE_URL}/ubbthreads.php?ubb=newpost&Board={$Board}" rel="nofollow">
2) I find another SEO problem today:
{if $postrow[topic].announce}
<a href="{$config.BASE_URL}/ubbthreads.php{$var_start}ubb{$var_eq}{$mode}{$var_sep}Number{$var_eq}{$postrow[topic].Number}{$var_sep}an{$var_eq}{$postrow[topic].announce}{$var_sep}page{$var_eq}{$page}{$postrow[topic].goto}">{$postrow[topic].Subject}</a>
{else}
<a href="{$config.BASE_URL}/ubbthreads.php{$var_start}ubb{$var_eq}{$mode}{$var_sep}Number{$var_eq}{$postrow[topic].Number}{$var_sep}page{$var_eq}{$page}{$postrow[topic].goto}">{$postrow[topic].Subject}</a>
{/if}
I originally thought that the variable "page" is actually the page number of the thread, but discovered today that it is the page number of the thread list pages. I changed {$page} to 1 as workaround on my live board, but this will break the Index link on the thread pages.