Then you will need to contact the people running the VPN or do a Wireshark trace to see what's up. I can't just guess at the issue.
Typically it could be
Time/Date
DNS
Gateway
Proxy
In that order, last would be a firmware upgrade. But:
If firmware was the problem it would not have worked before your side made the change, same goes for time/date, so they are out.
That leaves DNS/Gateway/Proxy, but the 8.8.8.8 got rid of that too.
So Gateway/Proxy, and of course you have a VPN/Firewall, which is firmly your end and I can't at all begin troubleshooting that. Best bet is to have someone good at Networking do a Wireshark trace.