Upon investigation, it was found that the certificate had expired earlier than expected and that the API private/public key must not have been refreshed either. To resolve this issue, follow the below steps:

1. Unregister the Edge Relay server by using the command 'ofcedgecfg.exe --cmd unreg --server --port --pwd' to ensure that it is disconnected from the ApexOne server.
2. Renew the certificate by using the command 'ofcedgecfg.exe --cmd renewcert --opacertpwd <OsceOPA cert password> [--keeprootca]'.
3. Register the Edge Relay server again.

After following these steps, the Edge Relay server should be successfully registered with the ApexOne server.