Urgent help needed!

[Barcham]

I'm receiving this error when trying to start a new thread.

Forbidden

You don't have permission to access /newreply.php on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

This is with my Admin user. One of my users was trying to make a post and received this error so I tried to make the same post and received the same result. This is a relatively long post with a few images and a couple of links. It is nothing overly complex and the exact same post is on another board running the same version of vbulletin.

I've noticed that if I trim down the length of the post, it seems to go through ok. It does not exceed the character limit nor the image limit set on the board. I even receive this error when trying to preview the post and if I try it a few times in a row, eventually the board crashes and takes around a half hour to come back up.

This is a new board I am setting up so there are only a few test posts on it so far and I have to get this fixed before really starting to open the board up to members.

Thanks to any help anyone can shed on this bug. It really has me stumped.