Something as simple as logging out of the forums, rebooting your pc, then relog into the forums. Try this because it worked for me today.
Just as a side note: I had tried this back when I first experienced the problem with trying to access my other characters on the forum, but it did not work then. So something changed between then and today. Might it be because today is maintenance Tuesday? Not sure, but now I can see all my characters but I had to actually log out of the forums. For just an extra step, I also rebooted my pc and then logged back into the forums and that’s when I saw all my other characters listed.