There are two separate issues.
One is caused by some conflict when you have 2 instances of 3dsMax running. Sometimes, on rare occasions, when you shut down one instance of Max and other one right after that, they will both try to write out settings and corrupt the file. This one is very rate though.
Other one is trivial. Some recent Max versions have a bug where some of the setting files are saved by default inside of location where user account doesn't have write permissions by default. Solving this is easy - just go over every single tab of Customize User Interface window, click save, and save settings to some location that your user account has write access to, such as your documents folder. Once you save them, Max will always read them from there. Then, your setting should not keep resetting anymore.