Starting I believe on Tuesday, I have had a couple workstations report that they were no longer able to open .MPP files from sharepoint libraries synced to onedrive for business. It would throw and error stating that either the file was from an old incompatible version of project or that the file path could not be found. After much research and troubleshooting I found that this was hit and miss. I've narrowed down the issue to being that if the file path and name in the synced directory is more than about 140 characters, Project cannot open it. However, if it is below that, it opens fine.
These directories have been synced forever and it is just now an issue, which leads me to believe this is from an office update (both users are on 2016 Pro Plus with first release). Users are on Windows 7 pro and Windows 10 pro.
Can you share the exact Project and Office version you now see Kylan - and I can take a look and see if I repro. Are you on First Release for Deferred Channel, July 12, 2016 Version 1605 (Build 6965.2066)? Best regards, and congratulations on being the first post in this group!
"Assume that you create an .mpp file in a SharePoint document library and then synchronize the library to the local computer by using OneDrive, and the file path is longer than 130 characters. In this situation, the .mpp file can’t be opened in Project 2016."