That is an encoded (signed) JWT being sent in response to the user info request that the FusionAuth OIDC identity provider is making.

This is technically allowed in the OIDC spec, but we do not currently support this response type.

Per spec, the endpoint should support a JSON response which is the default unless the client requests a signed or encrypted response body.

I would look at how your client is registered and see if it is asking for a JWT userinfo response at that time, and change it to be a normal JSON response. You could also file an issue detailing your needs for FusionAuth to support this user info response type.

If that isn't an option, you could also look at using a SAML Identity Provider if the remote identity source supports that.