Not sure how a screengrab of a successful VPN connection with the old firmware followed by a screengrab of an unsuccessful VPN connection with the new firmware is going to help much?

A screen grab will not provide all of the parameters that might effect it either. It's just a way of putting you off.

Sending a sanitised router config will probably be the quickest way to resolve it, there's all sorts of settings that could affect VPN functionality.

Please provide any information you can though, there are other situational factors like some issues could be specific to the type of Internet connection, the ISP, MTU, VPN client settings.

I use the 3.8.2.3_BT firmware on my 2860ac at home and that hasn't had any problems with dial in VPN from my usage of it.