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).