Thanks for your reply.
There is one Corporate network where the server now hangs.
From within the corporate network, in order to get to the internet, you must configure proxy settings in your browser (e.g., Internet Options on a Wintel client). Without these proxy settings, you won't go anywhere.
I was thinking maybe this was why I'm having connectivity issues. I'm not so sure now.
On a whim, I changed from static to DHCP and now I can PuTTY in, ping to/fro and even reach ISPConfig using the dynamic IP assigned to the server.
Then, I edited /etc/network/interfaces and reverted back to my static IP and now it doesn't work again. Hmmmmmm.....