With declarative pipeline

ansiblePlaybook([ 
             become: false, 
             colorized: true, 
             installation: 'ansible', 
             extraVars: [
                login: 'mylogin',
                secret_key: [value: 'g4dfKWENpeF6pY05', hidden: true]
              ],                       
                 playbook: 'src/main/ansible/prova_parametri.yml'
            ])

Error:
Also: org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: 14664390-9354-402a-9ae2-cdc3c89cdb1f
java.lang.ClassCastException: class java.lang.String cannot be cast to class hudson.util.Secret (java.lang.String is in module java.base of loader 'bootstrap'; hudson.util.Secret is in unnamed module of loader org.eclipse.jetty.webapp.WebAppClassLoader @76908cc0)
at org.jenkinsci.plugins.ansible.workflow.AnsiblePlaybookStep$AnsiblePlaybookExecution.convertExtraVars(AnsiblePlaybookStep.java:390)
at org.jenkinsci.plugins.ansible.workflow.AnsiblePlaybookStep$AnsiblePlaybookExecution.run(AnsiblePlaybookStep.java:422)
at org.jenkinsci.plugins.ansible.workflow.AnsiblePlaybookStep$AnsiblePlaybookExecution.run(AnsiblePlaybookStep.java:352)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1$1.call(AbstractSynchronousNonBlockingStepExecution.java:47)
at hudson.security.ACL.impersonate2(ACL.java:451)
at hudson.security.ACL.impersonate(ACL.java:463)
at org.jenkinsci.plugins.workflow.steps.AbstractSynchronousNonBlockingStepExecution$1.run(AbstractSynchronousNonBlockingStepExecution.java:44)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:829)