In the logs of our instance we have a lot of warnings from the Pipeline SCM API for Blue Ocean plugin, like that:
The corresponding code line is https://github.com/jenkinsci/blueocean-plugin/blob/977cf87fd07924eb7eb6e0f7854e5d739da1539c/blueocean-pipeline-scm-api/src/main/java/io/jenkins/blueocean/rest/impl/pipeline/credential/BlueOceanCredentialsProvider.java#L95
As an administrator I can't assign the problem correctly and it would be of help if the warning message would give details about that missing piece.
I am aware that we cannot print the user name into the logs but the credentialsID would help to determine where an admin needs to change a thing to avoid those warnings.
So here is my suggestion for the log line:
logger.warn("BlueOceanCredentialsProvider#getCredentials(): Username attached to credential with id: " + prop.getId() + " can not be found");