Can't issue refund odd error message.

[Hecateknickknack]

I'm trying to issue a full refund to a customer but everyone I try I get this error message:

{"success":"false","reason":"4001"}

I looked it up but can't find anything on what this means. I've looked over the refund multiple times and can't find anything wrong.

What's going on?

[NanaLetha]

If you are using the app, switch to a browser.

[Harajuku]

I've been trying for 3 days to issue a refund to a customer with no success. I'm getting the "Uh-oh" screen every time.

Using a browser, not the app, btw.