MyBB Community Forums

what is this? and how do i get rid of it? (the bottom thing, it also shows when i log out and in of my forum)

1) What is this?
There is no core file called 'pm.php' (or at least I don't see it), the error message is telling you the server failed to find it.

2) How do i get rid of it?
Stop requesting it.
Your problem is almost certainly related to a plugin, so (I suggest you) start looking at all your pligins which are related to PM's.

It's the AJAX PM notification plugin I think, isn't part of MyBB.