So, am I not alone in experiencing this annoying new behaviour?
It's nothing to do with the security alert.
The forums migrated to a new 'single-sign-on' framework a couple of weeks ago, in part to make it easier for users to move between different areas of Adobe's website (community help, my account, forums, etc.) and in part to reduce the amount of spam we get by adding an email verification step.
There are some problems with particular browsers not passing the cookie information across, which means the 'remember me' option doesn't stick. It is being worked on.
The Forum Comments section is already home to a number of threads, please post there if you have additional questions.