I don't know why. It's something related to your specific usage case. Many other people use it daily without issue and I don't find anything wrong using it myself either. You're going to have to do a deeper investigation, and be more descriptive/specific on how to recreate this, or I can't do anything about it.
1. What happens if you log out from the forum and log back in?
2. What happens if you clear your cookies in your browser?
3. What happens if you close your browser, reopen it, and try to access it again?
4. What happens if you try another browser?
5. What browser are you using?
6. Are cookies enabled in your browser?
7. When you get that message, can you access any other account restricted areas on the site such as the translation submission form
8. Are you logging in via the main forum page, or RHDN login page, or you don't have to login because a cookie remembered you?
9. What type of login are you using when you do login (1 hour, 1 day, Forever etc.)?
What specific URLs, buttons, and steps does it take to make it happen? Can you clear the condition via one or more of the methods above?