Checked all that but as I am running against a WAS that is Windows based the /opt doesn't work. Moreover, there is a problem with setting the HQ_JAVA_HOME variable in that environment. That's needed to get the correct IBM Java, but it does not really work and I have my doubts about the remaining parts of the installation, even though the windows agent manages to connect.
Plugin error: plugin class not found: com.ibm.websphere.management.exception.connectorexception (invalid classpath or corrupt plugin.jar)
It doesn't at this point, tell me what the path was that it tried. The plugin is apparently in the pdk/plugins directory where it is expected to be.