On June 13, 2020 I started getting emails from project owners in our PWA site that they could not edit their projects either through the browser or through their desktop Project application. In the browser the Edit button is grayed out. In Project, they get a popup error message about the server being in read-only mode. Not all users are experiencing this issue, but the majority are. I am an administrator on the site, but I created a test user account with the same permissions and tried to edit a test project and experienced the same results - Not able to edit.
I opened a ticket with MS and have talked to three different support people over the last 2 weeks but no one has any idea what is wrong. All users (those who can edit and those who cannot) are in the Project Managers and Portfolio Viewers groups and My Organization and My Projects are the categories associated with these two groups. I use the default security templates - no customizations.