HTTP 500 when trying to use pinafore.social

Hi, I’m trying to get the right configuration for CORSplug but for this, I will need a working external frontend to test this, so I’ve chosen https://pinafore.social.

When I try to connect to my instance, I get redirected to the OAuth login, but when I submit the credentials, I get the following HTTP 500 on my server:

[error] #PID<0.15955.0> running Pleroma.Web.Endpoint (cowboy_protocol) terminated
Server: communicating.cypherpunk.observer:80 (http)
Request: POST /oauth/token
** (exit) an exception was raised:
    ** (ArgumentError) non-alphabet digit found: "%" (byte 37)
        (elixir) lib/base.ex:1002: Base.dec64url/1
        (elixir) lib/base.ex:1089: Base.do_decode64url/2
        (pleroma) lib/pleroma/web/oauth/oauth_controller.ex:148: Pleroma.Web.OAuth.OAuthController.fix_padding/1
        (pleroma) lib/pleroma/web/oauth/oauth_controller.ex:73: Pleroma.Web.OAuth.OAuthController.token_exchange/2
        (pleroma) lib/pleroma/web/oauth/oauth_controller.ex:1: Pleroma.Web.OAuth.OAuthController.action/2
        (pleroma) lib/pleroma/web/oauth/oauth_controller.ex:1: Pleroma.Web.OAuth.OAuthController.phoenix_controller_pipeline/2
        (pleroma) lib/pleroma/web/endpoint.ex:1: Pleroma.Web.Endpoint.instrument/4
        (phoenix) lib/phoenix/router.ex:278: Phoenix.Router.__call__/1