This seems to be a common problem. I have installed NextCloud 18.0.0.10 on CentOS 7.3 using NGINX and a Postgres database following the standard instructions. Everything seems to go as designed: database is created (on separate server) and I can see in the database that the “admin” user has been created. The “admin” user files have all been created properly; the config file has been created properly.
The only problem is that the “admin” user cannot log in. No error is shown and there is nothing in any log showing up. The login page simply does nothing.