I was trying to setup Shibboleth SP with our Gluu Server which is used the Shibboleth IDP. For no reason, I was stuck at this situation. After speending few hours I was able to detect the issue and solved the problem. This may help you too.
2022-06-10 01:25:08 ERROR XMLTooling.CredentialResolver.Chaining : caught exception processing embedded CredentialResolver element: Unable to load private key from file (/etc/certs/domain.key).
2022-06-10 01:25:08 INFO XMLTooling.CredentialResolver.Chaining : building CredentialResolver of type File
2022-06-10 01:25:08 INFO XMLTooling.SecurityHelper : loading private key from file (/etc/certs/domain.key)