[resolved] Visual Studio does not find a namespace from the project

I am working on a project for quite some time and all of a sudden, the following error appeared in Visual Studio (Mac):

The type or namespace name 'Pun' does not exists in the namespace 'Photon'.

I checked that PhotonUnityNetworking is still where it is supposed to be (Photon.Chat and Photon.Realtime are still found). Visual Studio is up to date (8.5.4).

Unity doesn’t seem to have any such problems. The project is still built in the editor and behaves as expected. The namespace seems to just miss in Visual Studio. I have no idea what I did prior to losing the namespace.

I remembered what I did …
Some time today I upgraded to 2019.3.12f1. After reverting to 2019.3.11f1 everything works fine again.