When you try to access data on an Automation Engine container directly from a Mac client (for example: by double clicking an ArtPro file in the Pilot to open it), it's not possible to reuse an existing AFP mount to that container if it resides on a host having underscore (_) characters in its name.
On MAC 0SX 10.6.8, when you double click the ArtPro file, the system requests to mount an SMB share, although an AFP Mount already exists. ArtPro, then per design will use the SMB mount over the AFP to open the file, as there are now two mounts of the same share. This is by design correct behavior for ArtPro. When this happens the file will open, but then the shuttling will not work correctly as the Job ID cannot be correctly retrieved.
On MAC OSX 10.5.8, even when mounting via AFP and SMB, the opening from the ArtPro within the Pilot is not possible. You do not even get that far.
Work-around: Do not open ArtPro files by double clicking in the Pilot. Open the ArtPro file by using the Finder.