Retired techie here. When we interact with a web site, we engage in what is called a "session". The web server will maintain memory of us for the duration of that session, including the fact that we have logged in, who we are, etc. The session will expire (time out) if we have not interacted with the server for a specified amount of time (20 minutes is the default time-out). Sitting on a page and typing does not interract with the server, just our own computers. So if we take too long to type our posts, we hit the time-out, are "logged out" and now "forbidden" to post because only logged-in users are allowed to post. It's not a bug. It's just how things work on the World Wide Web.
Bottom line: Don't take too long typing a post here on this or any web site. As several people have recommended, you should compose your magnum opus beforehand, then copy-paste it into the text box to submit.