(and FWIW I think the error isn’t the JWK, or the URL, your logs look like the file gets loaded fine, the issue is the JWK isn’t formatted correctly according to the non-standard way in which Tyk uses it - there’s a reason it’s not documented!)
It would ultimately be much more effective to use OpenID Connect instead of a JWK hack. There’s no guarantee we’re going to keep this capability around, because it doesn’t follow the JWK standard.