>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.