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Same error, just pops out of nowhere, I was working with a build I had manually downgraded to 2018 from 2019 and this error just appeared, first...
As suggested above, reseting the layout on 2018.4.8f1 solved the issue for me
Same issue with Unity 2019.3.3f1 "Visual Studio Code Editor" version 1.1.4 bugged my existing project I had the "Generate all .csproj files"...