lnurl-pay returns error 400 for both Phoenix and Breez

Describe the bug When testing LNURL-pay with the same instance as in #3054 it returns error code 400. While it is likely it’s related to the problem there, if that is the case, the error code should be 5xy not 400 because it’s a server problem. It is possible that it’s unrelated, so I suggest to resolve #3054 first to find out. There are also no relevant logs after it fails.

To Reproduce the bug Steps to reproduce the reported bug:

1. Setup LNURL-pay with comment allowed (other settings are default)
2. Create an invoice
3. Try to pay the invoice from Breez/Phoenix, enter Hello world into comment
4. See error

Expected behavior

Either the transaction succeeds or, if there’s actually a problem with LN implementation, error 5xy is returned. 500 seems to be the most reasonable. The details of the problem are logged.

Your BTCPay Environment (please complete the following information):

• BTCPay Server Version: 1.3.1
• Deployment Method: Debian package
• Browser: irrelevant (not a browser)

Logs (if applicable)

None/missing

Setup Parameters

LN node: Eclair Store Lightning settings: Enable LNURL: true LNURL Classic Mode: true LNURL enabled for standard invoices: true Do not offer BOLT11 for standard invoices: false Allow payee to pass a comment: true

[Kixunil]

Don’t have much time lately but will try to check in upcoming days.

[Kixunil]

Sorry for it taking such a long time. I do NOT have shitcoins enabled, nevertheless I tested with 1.3.7 and still same error. Unfortunately I can’t upgrade to 1.4.* because of build issue. Looks like Microsoft messed up something in their official dotnet packages. Going to investigate.

Figured out their funny versioning, will test newest version and report.