BTCPay is expecting you to access this website from https://mainnet.demo.btcpayserver.org/. If you want to change this expectation:
- Either starts BTCPay with --externalurl https://mainnet.demo.btcpayserver.org/
- Or, if using docker-compose deployment, setting environment variable BTCPAY_PROTOCOL=https and BTCPAY_HOST=mainnet.demo.btcpayserver.org