OAuth refresh grant returns 500 internal server error

The openid configuration available at the .well-wknow iri for the NSS identity provider (e.g. https://solid.zwifi.eu/.well-known/openid-configuration) indicates support for the refresh_token grant. However, when issuing a refresh token request that should be valid (see below), the response is Internal Server Error.

curl --location --request POST 'https://solid.zwifi.eu/token' \
--header 'Authorization: Basic <client id:client secret>' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=refresh_token' \
--data-urlencode 'refresh_token=<refresh token>'

Steps to reproduce:

• Get a client ID, secret and a refresh token. That may be done by inspecting the dynamic registration of a client when logging in with an app for instance.
• Use the credentials to fill in the request above (client ID and secret should be base64-encoded)

Expected result

A 200 response with a JSON object in the body containing a new, valid access token.

Observed result

An internal error

Should this issue be reported here in the NSS repo, or in https://github.com/solid/oidc-auth-manager ?

NB: This is kind of related to https://github.com/solid/node-solid-server/issues/641

Start at the /token endpoint and trace how the refresh_token grant is handled in this repository, using the supplied curl request against the OpenID configuration example. Compare the behavior with the related issue and the oidc-auth-manager repository; done means a valid request returns HTTP 200 with a new access token JSON object instead of an internal error.