Same issue here with a project I switched from 5.3 to 5.4
I deleted everything in “C:\Users"UserName"\AppData\Local\UnrealEngine
I also deleted “DerivedDataCache”, “Intermediate” and “Saved” in my project.
But as soon as I restart the UE, it loads the default layout and there’s a content browser floating window.
I have another project on 5.4 and it works fine.