Thanks @Sibz9000 this hint totally help me with the same error. This is while using Unity 2020.2.0b1.2959. In my case the error seems to be triggered by me renaming the project folder. However, did exactly what you suggested and I can confirm it works for me as well. Otherwise I would have been locked out from the project. Huge thanks for sharing.
( As a side-note, I tried all the suggestions in this other thread without any success → Can not Launch Unity Due to "Failed to load window layout" )