[MadScientist]

I had the same problem when I upgraded my server to windows 10 - my HTPC could no longer access the shares.

It turned out that the guest account status is turned off in Win 10 as a security measure. When I turned it back on everything was as before.

Have a look at the Guest account status using the Local Group Policy Editor
Run gpedit.msc and then navigate to:

Computer Configuration - Windows Settings - Security Settings - Local Policies - Security Options - Account: Guest account status

Make sure it is enabled