Version 2.9.1.1 fails to build some Qt projects

One of our Qt apps stopped building its debug version this week.

Error message:

C:\Users\Bar\AppData\Local\QtMsBuild\qt_globals.targets(786,7): error MSB4184: The expression "[System.IO.File]::ReadAllText(C:\Users\Bar\AppData\Local\QtMsBuild\qt.natvis.xml)" cannot be evaluated. Could not find file 'C:\Users\Bar\AppData\Local\QtMsBuild\qt.natvis.xml'.

 I've attached a portion of a detailed build log.

It appears that in file "qt_globals.targets" for target QtNatvis, QtVersionMajor is not set. I don't know what to do about that. It is the only project with this problem, our other Qt projects build.

Creating a new Qt Widget project also works. I can see substantive differences between our projects and a new project.

Is there a recommended path to upgrade a Qt project other than recreating one from scratch?