Microsoft Windows allows assigning a drive letter to a network computer or folder, creating a mapped drive. A mapped drive letter (such as Z) can be used in a file or folder path just like a local drive letter (such as C). But there is a catch: the drive mappings are established when a user logs in to the system - and a service is not logged in (even if it is associated with a user account).
Thus while Switch server is running without any user logged in, it cannot access mapped drives. You must ensure that all folders and file paths in the flow definitions are local drive paths or UNC paths (of the form \\server\volume\directory\file) rather than mapped drive paths. This holds for all paths including user-managed folders and references to files in configurator properties.