Set 1:
a) export some valid, already working, certificate from another domain member computer and tried it on the problematic workgroup member (import into Local Computer certificate store). This would isolate problems on the workgroup computer.
b) make sure root CAs of the certificates' chains are trusted on the workgroup client
Set 2:
a) in AD, create an empty computer account and ensure it has the dNSHostName attribute. Make sure the object is enabled.
a.2) potentially, the empty password of the newly created computer may pose some problems, so I might go for ADSI Edit and would Reset Password for the computer account
b) add the computer account to the GlobalGroup that is specified in the NPS policy
c) issue a certificate for the computer account with Subject + SAN containing the dNSHostName value preciselly
d) make sure root CA is trusted on the workgroup client
e) try it on the workgroup client, import Local Computer certificate store
Set 3:
a) create a new user account with some UPN, set password, add to the group
b) issue certificate for the user and import on the workgroup client into user's profile
c) make sure RootCA is trusted by the Computer! Note that CRLs are validated by the computer profile, so make sure always that the computer (not just the user) trusts the root CA
ondrej.
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