The plist is corrupt or could not be read for some reason. Try running these commands:
plutil -lint /Library/Preferences/Xsan/config.plist
ls -leO /Library/Preferences/Xsan/config.plist
The first command checks the file type. It should be "XML document text" or "Apple binary property list". The binary plist format is less likely for this file, but still valid. If it's something else, like "Rich Text Format", that would be a problem.
The second command checks the plist for syntax errors and should point out where the problem is if there is one.
The last command just shows the permissions on the file. I don't think the problem is there. Since xsand runs as root, it should always be able to at least read the file. Compare this to another system where you aren't seeing the error.