In the course of doing a CLI upgrade from 2.2.6 to 2.3.1, the result of running
php bin/magento setup:upgrade was the message:
secret key size should be SODIUM_CRYPTO_AEAD_CHACHA20POLY1305_IETF_KEYBYTES bytes
No particular issue occurred when I ran composer.
Attempting to connect as admin or simply running magento as a visitor yields a similar message. So use is completely blocked.
I have no idea how to diagnose the issue and what should be done to resolve it. Any help would be very welcome.