The acutall error is "invalid session" as reefrunner pointed out. You can get this error a few ways and some of them have nothing to do with you or the site.
Let me give you a little background to this error and why it happens then Ill give you a few ways I have seen it occur. WHen you login your IP address and your username are hashed together in such a way a special session code is created. This code is stored as a cookie on your system and in the database on the server here. Every time you post durring your visit your session id is checked for validity. If it matches your allowed to post. If it does not match you get the error "Invalid Session". The only way it can not match is if your IP address changes while your still connected to the site OR you login to the site from a second computer or second browser window. This IP checking is acutally a security measure to keep people from spoofing your cookie and gaining access to the site as you. Now for regular users all they could really do is take over your account and post as you. But for advisors and admins having a secure account is VERY important.
First way. Your IP address (this is the computer address you get when you signon to the internet) changes while your connected to the website. If this happens you have no control over it as its something your ISP is doing. Who do you use to connect to the internet to?
Second way. You open a new window and visit the site in a second window while your typing your message in the first. Every time you login you get a new cookie and a new session. If you connect to the site twice (ie open a new window and visit the site in the second window) your first session from the first window is made invalid because you have a sesson now in a second window. When you go to post a message via the first window you will get that error.
There are some other ways at getting this error but those are the most popular methods. Do you practice the second way at all or do you never open more than one browser window when using the site?
although it has not been necessary since changing ISPs.
I belive you had used AOL for the longest time and now a different ISP. AOL is the biggest problem child for the first method of getting this error. They will use proxy servers and they will loadbalance them in such a way its possible for them to change your proxy server in the background while your connected. When they do this your IP changes and thus your session is made invalid.
To help track down why you get this error we need to know EXACTLY what your doing when your on the site and you get that error. I dont care what forum your in or what the contend of the message your typing. What we do want to know is if you have other browser windows open on one pc
or on mulitple pc
's. do you often leave the site up and leave only to come back a long time later? Etc. You dont get this error every time thus you must be doing something different when you do get this error or its your ISP thats doing something with out your knowlege.