Steps to reproduce the problem:

1. Access https://www.phpbb.com/community
2. Make sure you are NOT logged in to forum.
3. Select a topic for which you'd like to post a reply.
4. Click "Post Reply" button.
5. Enter you login credentials into the phpbb's login box, and press "Login" button.

You'll get the error message:

"The forum you selected does not exist."

First, I had observed the problem on one of my phpbb installations.
Then, I had confirmed the problem on two other phpbb forums.
The same problem occurs on phpbb.com forum itself.