I experienced the same problem after upgrading from NC 24 to NC 25. It’s odd that the documentation is so sparse, given that not setting this variable prevents the software from running.
However, I was able to solve it by setting the secret variable in config.php to the output of:
openssl rand -hex 64
However, after doing this, I had to authenticate all my apps and devices to my Nextcloud instance again. Also, I had to delete any NC-related cookies in web browsers. So be prepared to spend some time after getting NC to run again…