This is regarding a fresh install of Grav v1.7.0-rc.8 - Admin v1.10.0-rc.8:
When a new account is added to admin, and it's fields populated, trying to save it generates a 404 error. Then an attempt to log in using the new account's username and password fails with "Login failed...". Logging in with the initial admin account works correctly. This is the case on both my local development server and on the production server.
In user/accounts a new yaml file is created with what looks like a 32 digit hex number as its name (e.g. 'b5ec29f160ca2a63e70f08d39f6792bb.yaml'). If this file name is used as the username or if the yaml file name is changed to intended username everything works correctly. So I assume the 404 is because admin is looking for a yaml file with the correct username and at login admin expects the numeric name.
I'm guessing this is a bug and needs to be reported as such. But I thought I'd document this behavior here should others run into it.