This has been happening to me for the past several months (well before the forum update last month). I eventually get in, like the OP said.
I have also noticed for the last few months that if I am logged into the Forums and go to WM, I have to re-log in and it gives me the above error (but still lets me in). Previously, I could log in once and go back and forth between forums and WM without issue. Now I log into the forums no problem, but will go to WM and get the error. I also get the error if I go straight to WM (not going to and logging in the Forums first).