The problems:
1. I tried to submit my post and got the following error message:
Access Denied
You tried to enter an area where you don't have access
Continue...

Then I was logged out either automatically or by clicking the link "Continue...". I couldn't go back to my un-submitted post.

2. When I move away from the forum page I seem to get logged out.

Why?
If you are a forum member with access rights, this could mean that you have been logged out by the board due to the login timeout. If you do not check the Auto login box when you log into the forum, the board will only keep you logged in for a preset time (60 minutes). This prevents misuse of your account by anyone else. To stay logged in, check the box during login. This is not recommended if you access the board from a shared computer, e.g. library, internet cafe, university cluster, etc.

If you lost your un-submitted post after the automatic logout, you cannot got the post back by clicking the Back button of your browser if you have been logged out. You need to log in again and then go Back. Then you will be able to access your unsaved post.