На форуме Autodesk написано что причина может быть в
Issue:When attempting to save UI customization files (*.cuix) in 3ds Max, the error "Can't write mouse" appears and the Save file menu does not open.
Causes:
Includes, but is not limited to:Out-of-date V-Ray version and dongle.
3D Connexion Space Enterprise mouse.Out-of-date iToo Software plugins Forest Pack and RailClone.
Operating System (OS) regional settings.
Solution:
Below is a collection of the most up-to-date solutions available.
The list is not complete, and the final solution may require one, or a combination of the following:
1 Run 3ds Max with Administrator privileges.
2 If using a non-English region setting in Windows OS, use a non-English launcher of 3ds Max.