For this error to happen, you must have:

• a frontend siteaccess in Symfony
• the backoffice in legacy mode
• Remember me enabled in both siteaccess

Having the described setup, if you login the frontend and try to log into the backoffice, you will get the following error (see full stack in the attached PDF):
..Core/Authentication/Token/RememberMeToken.phpat line 60

You cannot set this token to authenticated after creation

    public function setAuthenticated($authenticated) 
    {
	if ($authenticated) {
		throw new \LogicException('You cannot set this token to authenticated after creation.');
	}
        parent::setAuthenticated(false);
    }

Steps to Reproduce

1. Configure your frontend site to use remember me as described in Authentication documentation page
2. Configure Admin to use remember me settings, according to the instructions from site.ini
3. Log into the frotend
4. Try to log in the backend