The way a VPN works is that it sends ALL network traffic through it's secure tunnel on a specific port. In this case, since it's most likely being blocked, that's why Safari isn't working when your VPN is connected.

I cannot tell you where to look in your router because there are many different types. But you're looking for port forwarding.

Resolved
 

OK. I resolved it.

On my Westell DSL router, I added a service called "IPSEC ALG" which, apparently, is a dynamic function that forwards to Port 500. The weird part is this all used to work, and I never used to have this service enable to make it work.

Kind of weird, but I am glad to be connected to the Exchange server again.

Thanks for your assistance, yellow.

New User - 8 hours Spent, but Still Can't get VPN working
 

I'm a new Mac user (first week) and need the VPN for my job - so am screwed without it. I've tried using SHIMO (no dice), followed all steps on this board/thread, plus 2 others without luck. I'm running VPN 4.9.00 (0050) on a MacBook Pro 10.5.5, with 2.6 GHz Intel Core 2 Duo. Upon launching by clicking the icon, I get "Error 51: Unable to communicate with VPN subsystem. Please make sure that you have...". Upon following some command line instructions on the previous page, I get nearly identical output:

$ sudo kextload -t /System/Library/Extensions/CiscoVPN/CiscoVPN
kextload: /System/Library/Extensions/CiscoVPN/CiscoVPN: no such bundle file exists
can't add kernel extension /System/Library/Extensions/CiscoVPN/CiscoVPN (file access/permissions) (run kextload on this kext with -t for diagnostic output)

What the heck do I do?

That VPN client is old, I think.
I use vpnclient-darwin-4.9.01.0090-universal-k9-BETA

Also, I assume:

* you have the correct IP address, username, password and connection protocols
* you already tried re-starting your machine

I have seen this error, and re-starting Cisco VPN tends to fix it. You can re-start it using Terminal with the following:

sudo /System/Library/StartupItems/CiscoVPN/CiscoVPN restart

Hair on Fire, I owe you one. After downloading the latest client, I'm in.

Fantastic.

Thanks