BTCPay Server returns a call back url which when followed takes you to the actually invoice url. I’ve just made a change on my server to see if it work. I’ve basically done a rewrite to forward directly to the invoice url.
The app got a http status of 400. Maybe the server was a bit busy? Tor / VPN could also affect this.
Discussion
It seems to work from here. But it was worked before. The 400 status returns before anything else.