Proton had a sale and I wanted to try their services out, but they would not let me register while on Mullvad's VPN. Their support said to not use a VPN when registering.
This seems like a bit of a silly reason to not trust a company. There's a small mountain of companies that won't let you register if you're behind a VPN, or using a less-common e-mail provider (ie: protonmail).
It's annoying but it's a bog standard technique for avoiding spam or fraud.
Is there perhaps some type of KYC law where they operate? Either way I would think it would be best to explain right there on the page, why they can't do it.
I can buy expensive things to have delivered to a vacant house and run off in the night if I used a stolen credit card on a VPN, but a cheap digital service that can be cut off at anytime if there's fraud, can't handle a VPN.
AFAIK you can register on a VPN, or even Tor, if you sign up with a paid plan, but you cannot sign up a free account without revealing your identity in some way.
They do accept Bitcoin payment, but not Monero. I wonder if that's because Monero is illegal for them, or just because they didn't bother to.
It's annoying but it's a bog standard technique for avoiding spam or fraud.