With MBPM, when attempting to deploy or retrieve an existing process, the Designer displays an "internal error in the deployment service" error, and the Event Viewer logs a Deployment Service error "The given key was not present in the dictionary".
If you have added indexes to the process data table (the table with the name of your process), remove them before redeploying the process or retrieving the process. After you are finished, you can safely add the indexes back.
This is usually due to custom indexes on the process data table. By default, the Deployment Service only creates an index on the eFolderID column. When it attempts to update or read the table and other indexes are present, it throws the above error.