Active 4.5 issue with VPN

Product: MC9094
Active Sync version: 4.5

One of my customer is complaining that Active sync isn't working when using VPN (Nortel). After disconnecting a VPN the device syncs perfectly. Customer has removed and installed 4.5 multiple times with no luck.I also recommended to disable the firewall but no luck. They only wanted to create a  partnership. On the other hand all other devices work just fine.

Does anyone aware of a known active sync issue with VPN?

Thanks
Mustafa