>>No idea, I'll pay attention to the next time it happens....
>
>I have simulated it. I am not sure if this was the pattern but there was a fix needed.
>
>1. You access the main page
>2. You click on "Try it before creating an account"
>3. Go back
>4. Do a login with enter or with the click on "Log in"
>
>At that time, the URL was replaced by the previous transaction from the cached page. The same pattern again will make sure the login works as expected.

That worked, but when I closed the browser, it remembered I was logged in anonymously. I had to physically click login on the right of the main screen.