Explanation
The cause of the issue is A. The certificate based on the Follower DNS name is not present on the Leader. This means that the Leader does not have a certificate file that matches the Follower DNS name used in the seed request, and therefore cannot generate a valid seed file for the Follower. This results in a 500 error in the Seed Fetcher container log. To resolve the issue, you need to import a certificate with the Follower DNS name as the subject alt name on the Leader, and create a copy of the certificate file with a name that matches the Follower DNS name used in the seed request1.

Explanation
The correct answer is A. The certificate does not contain the Follower hostname as a Subject Alternative Name (SAN). Generate a new certificate for the Follower.
A possible explanation is:
A Follower is a read-only node that replicates data from the Leader node in a Secrets Manager cluster. A Follower can serve requests from clients and applications that need to retrieve secrets or perform other read-only operations. To connect to a Follower with a REST call, the client or application needs to use the server certificate that was generated for the Follower during the installation process. The server certificate is used to establish a secure and trusted connection between the client or application and the Follower.
However, if the server certificate does not contain the Follower hostname as a Subject Alternative Name (SAN), the connection will fail with an SSL connection refused error. This is because the SAN is an extension of the X.509 certificate standard that allows the certificate to specify multiple hostnames or IP addresses that the certificate is valid for. If the Follower hostname is not included in the SAN, the client or application will not be able to verify the identity of the Follower, and will reject the connection.
To fix this problem, a new server certificate needs to be generated for the Follower, with the Follower hostname added to the SAN. The new certificate can be generated using the openssl command or another tool that supports the SAN extension. The new certificate also needs to be signed by the same certificate authority (CA) that signed the original certificate, and the CA certificate needs to be trusted by the client or application.
The new certificate then needs to be copied to the Follower node and configured in the nginx.conf file. The Follower node also needs to be restarted for the changes to take effect.
References = Secrets Manager Cluster Installation; Secrets Manager Cluster Configuration; Subject Alternative Name - Wikipedia