Invalid invoice id #0

Hello,

When adding the Tor URI in the module, when trying to make an invoice, I get the error:

`[ERROR] In modules/gateways/btcpay/createinvoice.php: Invalid invoice id #0` in ...onion/modules/gateways/btcpay/createinvoice.php

We use a .onion mirror in our WHMCS, if I pay the invoice on clearnet, everything works normally, but if I access our WHMCS through the .onion domain, when trying to pay the invoice, I receive the error above.

Noting that everything happens within the Tor network, with the .onion URI filled in. If I remove the Tor URI in the module settings, then I can pay the invoice normally, but it leaves my WHMCS .onion domain and takes me to the clearnet on the common BTCPay domain.

Unfortunately I was unable to diagnose the real problem. Could it be that he tries to POST something to the BTCPay .onion domain?

I couldn't find any error in the logs, maybe it can't find BTCPay within the Tor network

Start with modules/gateways/btcpay/createinvoice.php and reproduce invoice creation with the BTCPay Tor URI configured, comparing access through the .onion and clearnet domains. Check the available logs and the request path that produces “Invalid invoice id #0”; done means invoice creation and payment work while remaining within the Tor network.

Written by the indexing model from the issue text.