This problem has been described before in JENKINS-30593, which is flagged as duplicate but I cannot tell where the duplicate is that possibly offers a fix or explanation. I basically observe the same behavior as the author of that issue.

I can only deploy an EAR/WAR to my WAS server if I go to "Configure" and click on "Test Connection". This only works if I REMOVE the user ID and password of the server. After I run "Test Connection", I add the user ID and password back in and deployments work fine.

Each time I restart my Jenkins service, deployments do not work unless I do "Test Connection" without id/pw and then add them back in. It looks as if "Test Connection" is changing something in the running server that is required for subsequent deployments.

I did copy the required JAR files from the WAS server into /var/lib/jenkins/plugins/websphere-deployer/WEB-INF/lib. I did import the WAS server's cert into the JVM's trust store. And, again, the deployments work as expected, but only after hitting "Test Connection" without id/pw. There must be something in the plugin's code that is causing this odd behavior.