This morning I had the issue too, but it was when visiting a http link from the forum while usually I was using the https version.
I'm testing a change where the https cookie is stored with a different name than the http cookie (but which will also work in https, but stored non-securely as before).
If it works, I'll put that online tomorrow.
In case you lose your password, there is a link on the login page to reset it via email