Unresolved compilation problem?

All of a sudden, one of our programmers is getting the following error on any project. I have never seen it before. They are running a recent WPILib (before the release a few days ago; 2021.2.1 or 2021.2.2). I am wondering if anyone has any ideas or knows how to help troubleshoot?
Here is the error.

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
        RobotBase cannot be resolved

        at frc.robot.Main.main(Main.java:23)

Can you try updating to 2021.3.1? Changing the version in build.gradle should be enough. And make sure that you aren’t using dev builds.

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Thank you. I asked him to try that. I will report back when I hear how it went.

Edit: Just an update. That did not help. WPILib updated, yet the same build errors persisted. I recommended uninstalling and reinstalling. He ran into some hiccups reinstalling, but I think he will figure them out. There were permission errors on uninstall (locked files). So, our hypothesis is the two issues are related.