Receiving an error when trying to respond to a PM

this is the error message that I am getting ---

Not Acceptable

An appropriate representation of the requested resource /private.php could not be found on this server.

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

I'm getting that too.

Same here. Gave up on 2 different post yesterday. Oh well.

I am having this problem of the same error when replying to a thread (how fast can you push soft alloy - in CastBoolit sub-forum). Oddly I can obviously post a reply here however.