In the logs of our instance we have a lot of warnings from the Pipeline SCM API for Blue Ocean plugin, like that:

2021-05-20 18:17:06.018+0000 [id=11197769]      WARNING i.j.b.r.i.p.c.BlueOceanCredentialsProvider#getCredentials: BlueOceanCredentialsProvider#g
etCredentials(): Username attached to credentials can not be found

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");