Project not checked out to you anymore

[Jo]

A user opened up a project file in Pro, did nothing, and
closed Pro, and got the error:

Project not checked out to you anymore.
The project is no longer checked out to you on the Project
Server. To close the project, click OK. To save the
project as a file, click Cancel, and then save it as a
file on your computer.

I certainly haven't been checking in projects. Why would
he be getting this error?

Prior to opening the file in Pro, the user had accepted
some task updates in PWA, and it said there was an error
updating the MS Project file - but when he opened the file
in Pro, the tasks appear to have been updated correctly.