It's supposed to be lastpostinthread, not lastposttothread. DaniWeb doesn't seem to be experiencing this problem? Can you confirm that DaniWeb has the same bug? I'm not sure why it's doing what it's doing, but this should eliminate the problem:
To fix it, in functions.php:
Replace
[php]
'#<a ([^>]*)href' . preg_quote("=\"showthread.php?$session[sessionurl]t=") . '([0-9]+)' . preg_quote("&goto="). '([a-z]+)"#',[/php]
with
[php]
'#<a ([^>]*)href' . preg_quote("=\"showthread.php?$session[sessionurl]t=") . '([0-9]+)' . preg_quote("&goto=next"). '([a-z]+)"#',[/php]
Also, replace
[php]'<a \1href="\3tothread\2.html"', [/php]
with
[php]'<a \1href="next\3tothread\2.html"', [/php]
I have edited the first post to reflect this change.