I found an issue on my board, and since I can reproduce this on phpbb.com, I'm filing a bug report.
1. Browse to a topic as a guest.
2. Click the "Post Reply" button.
3. Login with your username and password.
4. After logging into the board, you are presented with an error message "The forum you selected does not exist." and the following URL: posting.php?amp%3Bf=64&%3Bt=2270781&mode=reply&sid=8ba2cd47b433142575cd3c49b5d8c6c7