Uncheck the external editor open in a separate windows option.
Before posting, please read how to report bug or request support effectively.
Bug reports without an attached log file are usually useless.
I set Matlab as external editor, and now I have no problems editing multiple files and saving them. However, the problem appeared when I used Open. I prefer to use internal editor for general purposes and edit Matlab files using Open command. What could be the problem? As I said, it fails to create a directory. I went to preferences, all reasonable boxes are checked.