Created a Custom Plugin jar, based on the instructions mentioned in https://docs.appian.com/suite/help/24.4/Custom_Smart_Service_Plug-ins.html . Deployed the jar in admin/plugins folder as per the instructions. The plugin does not load and show up Process Modeler. The server and Appian(on-prem) instances starts up normally.
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Apparently it failed to register the plugin. Check the tomca-stdout.log file, you'll see what went wrong