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.