As defined previously, sandboxed plugins are executed by the Dynamics 365 Sandbox Processing Service (Microsoft.Crm.Sandbox.WorkerProcess.exe) on the sandbox server. In order to debug a sandbox plugin, follow the same steps as this recipe and attach your debugger to the correct process.

You will also need to disable the auto-shutdown feature that shuts down the process if it is unresponsive for more than 30 seconds, which is usually the case when you are debugging. To do so, set the HKEY_LOCAL_MACHINESOFTWAREMicrosoftMSCRMSandboxDebugPlugins registry key to the value of 1.

For further details, visit https://msdn.microsoft.com/en-us/library/gg328574.aspx.