Recently had a problem and was unable to save MS Project files to O365 Sites (.mpp files). Turns out it was a known issue with Windows 8, and a hotfix was available.
When you open a Project file from a SharePoint document library, the file is sometimes opened as read-only when it should be read and write. In other cases, the project is sometimes opened as read and write but you can’t save changes. Note To fix this issue, you also need to install March 8, 2016, update for Office 2013 (KB3114816).
More information here https://support.microsoft.com/kb/2812235
How to save MS Project files to O365 Sites – Part 1
Here is a list of the ways our .mpp file can get to SharePoint.
- Saving an .mpp file using Save-as to a OneDrive for Business (SharePoint) document folder
- Copying to a SharePoint document library using drag and drop
- Copying the file to a local drive mapped to the document library
- Options for opening the file – click on the filename in the web view
- Options for opening the file – using the Edit option
- Options for opening the file – open from the local mapped drive
- Options for opening a file – from another document library that expects a different account than the logged on user
For a detailed breakdown of the items above and to find out related info, please visit this link https://blogs.technet.microsoft.com/projectsupport/2015/01/06/saving-projects-to-sharepointand-opening-them-again-part-1/.
I hope this helps.