Use of toString() on hudson.util.Secret from hudson.util.jelly.MorphTagLibrary$1.run(MorphTagLibrary.java:91). Prefer getPlainText() or getEncryptedValue() depending your needs. see https://jenkins.io/redirect/hudson.util.Secret/
Sadly one of the locations in the code where that log message is unhelpful. Nicolas didn't care though: https://github.com/jenkinsci/jenkins/pull/3668
As you filed this as a bug, what is the behavior you'd expect to see here? Because I can tell you already that this kind of invocation cannot be removed.
I'm not sure what I did to trigger it. Which is unfortunate.
This happens when a Secret field is shown on the UI in a control other than f:password.