I have had this problem for a while and thought it would go away, but it's still there.
When you work on a post, especially the photo forum and get tied up trying to upload a correction to one of your pictures you are trying to post, and say it takes you about 10 minutes to work it out. When you get it corrected and finally send the post, it comes back that you are not logged in. Then when you log in, you get and error message saying you need to report it to your site administrator. It does not return you to your post. I have learned to go back two clicks and can find the post I was working on and send it again and it accepts it. If you have 20 minutes or so of info you have been working on, it's kind of disheartning to do it over again. If I know I have been working a while, most of the time I will copy my work, so if I lose it, I can do a new post and paste it in.
Try it and see if it happens to you. I really don't know how long the timing interval is for this to happen, but I would say longer than 10 minutes.
Big Bad John & Runaround Sue