Tom, I have the same problem every now and again. It’s a little bit of a roundabout way, but my “fix” was to sign in, hover the cursor over the “Forums” button at the top, hover over the “Forums” button with the arrow to the right, and then click on ask a member. From there I click on a post (Shouldn’t matter which one), then where it says:
“Home / Forums / Ask A Member / ‘title of post’ ” towards the top, I click on forums in the section and it takes me to the main forums page instead of the sign-in page.