I have a bad thread on one of my Forums that has a strange date on it. It prevents the Forum from opening when you click on it. So I turned off the Forum from view while I fix it. You can go to the Forum directly by the direct link and still post on it. But if going through the bulletin board and clicking on the Forum, it won't show (it times out). How do I delete that file? If you click on the first thread, you'll see it has been deleted...but it's still there. Here is the direct link to the Forum:

http://www.linda-goodman.com/cgi-bin/ubb...+&number=16

Wow you really need to upgrade.
But anyway from memory.
From the control panel see if you can find the post to Prune it.
It may be deleted already.
If that is the case from the control panel re-index and see if the post goes away.
Also you could ftp to the site to see if the html file is there.

Looking at this more this may be a left over glitch from your previous problem. so maybe just re-indexing may solve the issue.
The post "NO" has a weird file name. It almost looks like a date then the actual file number.200904301655.html
So I would look for that file or 1655.html
I do recall that each forum had a separate folder and it had something called forumthreads.cgi which held the index of posts. Then there is a separate file for each post.

Just another FYI.
The longer you wait to upgrade,The more difficult it is going to be.
Right now the current version is 7.5.2 and will be 7.5.3 soon.
So currently you would need to upgrade in two steps.
First to classic 6.X then to Threads 7.x
Then Rick is currently working on a new release 8.X. So it will just get more difficult as time goes on.
And as we move on in versions more of us will have less memory recall on how to resolve issues like you have.

go into /ubb/Forum16/ and delete 200904301655.cgi. The run rebuild threads. Should fix it. That particular topic file is corrupted. While it might be fixable it's easier to delete the file.

Evidently the server died, timed out, or ran out of disc space while writing that file.