Seems found solution. 
inc/functions_forumlist.php find
if(my_strlen($lastpost_subject) > 25) - Change to 15
$lastpost_subject = my_substr($lastpost_subject, 0, 25)."..."; - Change to 15.
Then width is correct when is to long text in index page "Last Post".

inc/functions_forumlist.php find
if(my_strlen($lastpost_subject) > 25) - Change to 15
$lastpost_subject = my_substr($lastpost_subject, 0, 25)."..."; - Change to 15.
Then width is correct when is to long text in index page "Last Post".