[Mihnea Balta]

Does it say anything else in the script editor besides "cannot save MXS"? Is the path where you're trying to export writable? Are you trying to overwrite a read-only file, by any chance?

Try this:
- file -> export
- change the selection in the file type box. It doesn't matter what you change it to, just change it. For example if it's MXS, switch to MA.
- export the scene with the newly selected format
- file -> export again
- select MXS and try to export the scene now

This is a workaround for a Maya bug which appears after a certain combination of loading/unloading/using an export plug-in. If this is your problem, you only need to apply the above sequence once, it will work correctly after that.