SoundTrack Pro Can't save error

My Work flow:

Source video = ProRes HQ QuickTime file stored on a Fiber attached XSAN
I drop the Quicktime file (stereo audio) into Sound Track Pro.
-Adjust audio levels and do some mild dynamic compression

Choose "File/Save as" and ad an "a" at the end for the file to signify new audio levels

Confirm that the file needs to be flattened.

STP goes through the saving process and at the end I get "The Document "XXXXX" could not be saved as "XXXXFilenameIselectedXXX"

There seems to be no logic or pattern as to when or why this happens. Anyone got any input on this?

Thanks.

Seems unlikely as you are on an XSan, but have you got enough space? I only say that because of this article from Apple Support

When you save an audio file project as a QuickTime file in Soundtrack Pro, you may see an error that says the document could not be saved:
This happens when the drive you're saving to doesn't have enough free disk space to store the file. To resolve this issue, either free up space on the drive or save to another drive with enough available space.

Or, possibly more likely, something like this

When you save a Soundtrack Pro multitrack project (.stmp) file to an Xsan volume on a computer running Mac OS X 10.4 and Xsan 1.1, you may get the following error: "The document X could not be saved" (where "X" is the name of your multitrack project).

To avoid this, save multitrack projects to a local drive. Please note that you can save audio file projects to an Xsan server with Xsan 1.1 without issue with Mac OS X 10.4.

(See chapter 3 of the Soundtrack Pro User Manual for a description of the two project types).