Hi,
What do you exactly mean by "When I move the cable from the 1841 to the 1941?
Do you mean that webVPN stops working when you connect the two ISP at the same time or do you have it when you replace the ISP2 link with the ISP1 one?
I don't think that the certificate would be an issue even if you are running on a different IP. Your browser will complain but the portal should still be served.
I would first try to telnet to your portal IP on port 443 to see if 3WHS is completed. If it doesn't, maybe it is worth checking the ZBF config and logs.
If it works, maybe check that the router holds the private key and the certificate which is bound to your WebVPN gateway.
Regards,
Nicolas