SharePoint task list shows “read only” when opened in Project 2013

SharePoint Online has a nifty set of features for managing project tasks, but I don’t like its web interface. I prefer Microsoft Project (which makes me a bit of a heretic, but there you have it). SharePoint has a button that will take the current task list and give it to you as an .mpp file, then open it in Project so you can edit it. Sadly, this button only works in IE for Windows; it doesn’t work on my machine when I use Chrome, and of course since there’s no Project version for Mac OS X, it doesn’t work there either.

Anyway, I’ve been using this button a lot, but the other day it just quit working. I’d click the button, the .mpp would open, but Project would see it as read-only, so I couldn’t save edits back to the SharePoint site. I did some digging and found that other users had reported success opening the .mpp directly from the “recently used” file list, but that didn’t work for me. So I posted this thread. Meanwhile, as I waited for an answer, I tried a variety of things, none of which worked. Then, in a completely unrelated set of operations, I signed out of Office 365, changed my account password, and signed back in… and then it worked. Perhaps this fix will prove useful for future generations.