I'll have a look at the regfix to see if this makes a difference. I've also read the other thread you posted the link to but I don't it is the same issue as I can't get connected in the first place.
Windows 7 L2TP/IPsec Instructions
To add a necessary registry setting:
1. In the Start menu search box, type "regedit" and press ENTER
* You will be prompted to allow Administrator rights, click Yes.
2. Locate and click the registry subkey named HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent
3. On the Edit menu, point to New, and click DWORD Value
4. In the New Value #1 box, type "AssumeUDPEncapsulationContextOnSendRule" (this is case-sensitive and contains no spaces), and press ENTER.
5. Right-click AssumeUDPEncapsulationContextOnSendRule, and select Modify.
6. In the Value data box, type "2" and click OK
7. Reboot the computer
Sorry, i had not realised you had replied as i received no notification.
If this has worked previously with your equipment then i would use a new PCF file in your client. This would either mean removing the current one and just importing it from the original source you were provided with or having to contact the VPN Concentrator admins and had a new PCF mailed to you.
Have you turned on Logging in the client too? That can usually suggest credentials too.
I support this setup daily and Error 412 is almost always a corrupt PCF or invalid credentials being parsed between the Concentrator and the client, its never been an ISP problem but one at either end of the connection (be it connectivity due to firewalls or account variances (invalid or corrupt).