This specific user has never even used Classic. The switch to go back to classic is disabled for everybody. All profiles have access to Lightning Experience. All users get a Lightning landing page after logging in, except for one user, who lands in Classic. This happens on different devices, with different browsers.
After clicking the 'Switch to Lightning Experience' button, he gets to LE. But the next time he logs in, it's Classic again.
What can cause this? The user has the same profile as many other users.