[URGENT] blank page for last post

[nimasdj]

When I try th eLast Post link I get a blank page. I did enable SE-friendly url as my site supports RewriteMod
/thread-17-lastpost.html
for sure I tried this url and again got a blank page.
/showthread.php?tid=17&action=lastpost

but when I brose forums to threads, it works fine. why I am getting this blank page?
Does this cause by a bug in instqalled plugin? or it does not have anything with pluging?

I also added
$l['my_copyright'] = "blahh"; in global.lang.php
then in fotter template I edited the default footer to
<center>{$lang->powered_by} <a href="http://www.mybboard.net" target="_blank">MyBB{$mybbversion}</a>, &copy; 2002-{$copy_year} <a href="http://www.mybboard.net" target="_blank">MyBB Group</a>.<br />{$lang->my_copyright}<br /></center>

and it works fine on other pages, does this might cause the prblem in lastpost link?

or these al have nothing to do with th eproblem? please advice how to fix it.

Thanks,

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I reversed the customizations above and remove edit reason plugin and I still have th eblank problem with lastpost. why? any idea how to fix it?

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here I ntoced when I click lastpost, the link will be converted to something like:
/thread-36838-post-245834.html#pid245834
but in mine link will be still:
/thread-17-lastpost.html

why? what might cause this?

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well, when I open the page with FirFox I just see: ""
I looked all files I recall I edited for  at the start and found nothing all start with <?php.

can you tell me which files are called before showthread.php which runs to show lastpost?