After the old launcher updated to the new one, starting any of my profiles yields the following error:

The game directory is invalid or inaccessible.
Error details: The system cannot find the file specified
Filename on disk: .minecraft
Path: C:\Users\XXXXX\AppData\Roaming\.minecraft
Exists: directory

This folder exists. Removing it and trying again results in the folder being created again, and the same error.

Clearly it's already accessing this folder because that's where the launcher has its profiles stored, so I'm not sure what it's complaining about.