I would guess it could be one of the following issues:
1. Your firewall blocking PunkBusterA or PunkBusterB
2. Your Punkbuster installation needs updating.
3. You dont have PunkBusterA or PunkBusterB installed.
Run the PunkBuster install/repair tool and then use the PunkBuster auto updater to update your installation.
Both of which can be found here: https://www.teamukom.co.uk/downloads/ca ... punkbuster