Close your browser, restart it, and try again.
The problem is that you have some token (a piece of data, similar to a cookie) that is exchanged between your PC (or phone) and the Forum's host. It could be in/from some tab you opened days ago.
Entirely closing and re-starting the browser is the easiest way to get it to be "refreshed" (due to the creation of a new session).
You could try (for amusement's sake) intermediate baby-steps (such as clearing your browser's cache), but stop/restart is the simplest in terms of number-of-clicks and menu-hunting.
--dick