NetBox released the version 2.9.0 where after the upgrade the login failed with a Server error due to a change in the Authentication backend.
After the upgrade my installation came up with the error screen shown below:
The message to check missing required packages and WSGI service not restarted after upgrade might be a bit misleading.
The cause is a change in the Authentication backend, which is addressed now in version 2.9.1.
Some additional information are in Issue #5033.
To fix this issue, the line with REMOTE_AUTH_BACKEND needs to be changed from utilities.auth_backends.RemoteUserBackend to netbox.authentication.RemoteUserBackend in the file configuration.py.