I’m guessing you tried it, but try logging in without a VPN.

I’ve had trouble in the past from logging into accounts whilst my VPN has been active, likely as a security response from Mojang.

I did a little research, seems a lot of people use it to bypass, but I found this reddit discussion saying that you can get banned by Mojang for using VPN: https://www.reddit.com/r/Minecraft/comments/5vwve1/mojang_now_ban_accounts_for_vpn_usage/ although other comments say they’re unable to connect whilst VPN is active, and have to disable it.

It would be best if a lot of these companies start encouraging more security questions when people login with the rise of VPN usage, with especially the correct credentials, and with which the questions have to change monthly all as part of security measures against real hackers from a location and IP which doesn’t reflect your own.

Anyway, hope you find a resolution!