I am getting the same error when try to log out by redirecting the user-agent to the /logout endpoint.
I am passing the (URL-encoded) ID token as the value for id_token_hint as a parameter on the URL. The URL looks like:
https://dev-{...}.okta.com/oauth2/default/v1/logout?id_token_hint=%7B%27access_token%27%3A+%27{...}+%27scope%27%3A+%5B%27profile%27%2C+%27email%27%2C+%27openid%27%5D%2C+%27token_type%27%3A+%27Bearer%27%7D
(I have replaced sensitive information with {...})
I am confused because the error mentions a client_id parameter, but the API docs for /logout do not list client_id as a parameter.
It seems that Logout Redirect Page addresses a similar issue, but the solution was very specific to the poster’s C# implementation, and I am not familiar with C#.