Are you using Redis to manage your Sessions? If so, I had the same issue. It’s caused when your session handler isn’t configured to handle the traffic. You can usually reproduce the message my opening multiple tabs in quick succession.
Have a read through the Related GitHub Article linked below? You’ll notice that I took part in it.
Suffice to say that increasing my max_concurrency value in the env.php file solved the issue. The default value is “6” which is really small. The new value you replace it with will depend on what your web server can handle.
If you’re not sure, try increase it in small increments and see how you get on. Maybe change it from “6” to “12” to start with.