Looks like I am not the only one.
I did exactly what you did, troubleshoot the post paragraph by paragraph. When I came up on the problematic paragraph, I troubleshoot line by line, meaning I insert one line at a time until I insert the line that would give me that '404' error. Then I found the word 'from' to be the cause. If I put the word in quotes, then I could preview the post just fine, but if I remove the quotes, then I would receive that '404' error.
In post #25 of this discussion...
http://www.defence.pk/forums/military-aviation/41035-logic-behind-jf-17s-2.html
If I substitute the word 'between' with 'from' then I would get that '404' error. I sent the Webmaster a PM and that message got borked up if I do not quote it. The strange part is that it does not occur all the time. If I do something like 'Testing...from' and preview it, no error at all. So it seems like there is something else going on with forum software.