VPN for IPSec - router versus ethernet to modem

I have an odd problem than began yesterday morning. I can access work development servers using VPN. I have configured the Mac VPN software using the Cisco IPSec type. It has worked for the past two months on my new Linksys N900 (EA4500) router, wired or wireless. Suddenly yesterday morning, it no longer works through the router. Once I try to connect and then I close the VPN connection, all other traffic fails, e.g. no email, Dropbox, or browser connections work, until I restart the computer. The only way I can connect to our VPN is wired, directly to my DSL modem.

Oddly, I can use the wireless access point with the same VPN connection at other locations such as our remote office where we still need VPN to connect to our servers.

Should this be an issue with my router, the VPN software settings, priority of network connections? It seems as if the VPN connection is "working" but then no traffic actually gets through the tunnel. And yet I changed no settings on the router.

Any help would be greatly appreciated.