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/
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.