Ms Files Not able to Upload File

The error appear on screen is "The directory private://ms_files is not accessible".

When i create or add File Download Plans (admin/structure/ms_files/add), I am not able to use the "Upload the File here" field to upload file. After hitting Save button I got the above error message.

I have been using Private file system since last years so I am quite familiar with Private file system. Previously when I was using File Download Plans, it was able to upload file. Just recently I uploaded few modules, removed few unused modules from the server, and tweak some permissions at /admin/people/permissions , then the above error appear. However I don't think all these changes will affect the module and gives above error.

I am sure currently the site able to access Private file system and all folder within the Private file system.

Anyone having the same error or have any suggestion for me to fix the error will be highly appreciated.