It might have been a coincidence. Someone identified a weakness in the phpbb3 software and used it to begin attacking our site a few days ago, thanks in part to the fact that the server doesn't have a whole lot of RAM on it (yet) and can be easily overloaded through this technique. It's possible that you just happened to try to post in N&P when he was launching these attacks, which came and went.
Nope. It actually just seems to be the one thread. As I test I tried to post there again, got the 500 error. So I tried a new post in N&P and previewed, it worked fine. Went to another N&P thread, previewed a reply, it worked. I go back to the original thread, try to preview my reply...and I get a 500 error. Yet other people have been posting replies to that thread since I started getting the errors, so it just seems to be me for some reason.
At least when I tried copy/pasting my old message. I just rewrote another one (even shorter than my last two attempts, *sigh*) and it actually worked. So maybe there was something odd with the message or it was too long or something? I really have no idea what the deal was. But the problem seems to have passed.
EDIT: So I still had the text of the message giving me problems saved in a Notepad file and decided to preview it in Testing. Sure enough, a 500 server error. So it must have had something to do with the text of the message itself for some reason... The only thing different about it is that I had copy/pasted some text from a PDF file. When I took just that text out...it worked! So for whatever reason it was the text from the PDF that seemed to be causing issues. I have no idea why; it was eventually saved as a Notepad file and taken from there, surely that would have done away with any anti-copying code? I didn't even know PDFs could do that in the first place.
But that seems to have been the issue. Weird.
EDIT2: Okay, it was the quotation marks. They look different than the ones I see when I press the key. Replacing those fixed it. I guess that particular font for quotation marks or whatever that was translated over from the PDF made phpBB very unhappy.
"How can I wait unknowing?
This is the price of war,
We rise with noble intentions,
And we risk all that is pure..." - Angela & Jeff van Dyck, Forever (Rome: Total War)
"On and on, through the years,
The war continues on..." - Angela & Jeff van Dyck, We Are All One (Medieval 2: Total War)
"Courage is not the absence of fear, but rather the judgment that something else is more important than fear." - Ambrose Redmoon
"You either die a hero, or you live long enough to see yourself become the villain." - Harvey Dent, The Dark Knight