Navigation not working in Unity project: Cannot find declaration to go to

Well, this is peculiar. I have a complex Unity project. I double-click a .cs file, it launches Rider. Once open, I can navigate as expected; e.g. where a method is invoked, Navigate -> Go To Declaration or Usage, and it goes to the declaration.

Now I clone my Unity project. I open it in the same version of Unity. It builds and runs just fine. I launch rider by double-clicking the local copy of the same file, and once open, attempt the same navigation. This time I get a pop-up: "Cannot find declaration to go to". (FWIW, the declaration is in the same file about 25 lines down.) Pretty much nothing else works as expected. Rider is acting like a dumb text editor.

So: Two copies of the same project; same Unity version; same Rider version; different results. I'm at a loss how to diagnose and fix this. Advice?

Unity: 2019.3.13f1
Rider: 2020.3.4 (Build #RD-203.7717.10)

As for the Unity Rider Editor integration package, I've experimented with different versions with the same lack of success.
In the working project: Rider Editor V 1.1.4 (2019.3 verified)
In the non-working project I've tried V 1.1.4, V 3.0.5, V 3.0.6