Unable to Point Calibre to a new location

I am setting up Calibre on a new Windows 10 PC. Calibre is in Dropbox. I have tried pointing Calibre to the existing library and I keep getting the message that the library is invalid. My library has over 6000 books and the metadata files are there.

I've tried the Import/Export tool. The Switch Library tool and the Welcome Wizard. I have the same issues with all three. I've done this previously with multiple pcs so not sure exactly what the issue is. Can anyone guide me through this please?

Thanks