Payroll access in the Windows 10 app has been working from about a month after I first posted (feb). The one remaining annoyance is that the cookie seems to time out (if the Employees tab is inactive) and the only workaround I’ve found is to close the App and start it again.
The unresolved issue, as far as I am aware, is the need to (globally) enable 3rd party cookies in whatever browser you use to access payroll, not in the app. I consider this an unacceptable requirement and security risk on my computer, but Intuit think differently.
