OneNote cannot open OneNote main file saved in My Documents folder

I am not able to open existing main OneNote file save in my Host My Documents subfolder. I have to create new file in C:\...\Documents\folder. And, I am not able to import the old file (tabs). I had lots of content in my old file. Why can't I simply restore my old content? 

There has to be an issue with the overall architecture of OneNote. It should follow the same logic as .docx, xlsx, pptx. Instead there is no consolidated file that a user can simply backup, restore, open and work.