Hi everyone,
I apologize that we did not have a response for this post sooner. I did some playing around with adding a plugin to a project (using getnamo’s Hydra plugin in version 4.5.1 of the Engine built from source code) and tried packaging projects in various scenarios (development configuration, project created in different locations on different drives). If I placed the plugin in the project folder, I was able to package the game without any problems. If the plugin was placed into the Engine folder, then packaging failed. I saw the same results in a couple tests using the binary version of the Engine.
Would you be able to provide more information about what plugin you are trying to use, where you are placing the plugin, where the project and Engine are located, and the settings you are using when packaging?