Start9 node serves BTCPay server for LNbits for LN address. I have a separate server running where I run nginx to do a reverse proxy. I also use Cloudflare. This setup has worked for two years previously. The only thing different is a new node backend. Though the confusing part is it works depending on which wallet you're using to send.
Discussion
How did you backup and restore the LN node configs? Do you have the old node to compare configs on? Or maybe temporary failover to right meow?
start9 has a built in migration tool that copies the the lnd data directory. i was close / am close to just building my own node. you can migrate the lnd data directory yourself.
i wonder if it's a CORS error. i did a curl on https://pay.derekross.me/.well-known/lnurlp/pay and im not seeing the CORS header. it should be being applied via nginx as that config hasn't changed. i wonder if Start9 is removing it.