Has anything changed on your Active Directory Domain Controllers?
If you go to a machine, that is having this problem, open a command prompt and type "set" all by itself.
Scroll back up in the prompt window and you will see an entry "logon server = "
Verify on this ADC event logs that this ADC is not having any issues.
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Have you recently upgraded the domain/forest functional level?
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You can also verify:
- \\logonservername\netlogon\ do you see the login scripts?
- Open Active Directory Users ans Computers
-- Navigate to a user that is having problems and open properties
--- This user has a logon script assigned? And the logon script is valid (exists on logonserver\netlogon)?