If the name of any path or file involved in the conversion contains any characters other than letters and numbers (such as spaces, dashes, or underscores), rename the path or move the file to eliminate them; see §1.1.2 File, directory, and path names.
If you are using a network drive, move your files to a local drive and try running the same conversion there before asking for support. A network is inherently slower and less reliable than a local system, so you might have mysterious and intermittent problems when a heavily loaded network is used for file storage.
File names and paths valid, and the problem still exists? Next: Check your version of DITA2Go.