In oidc_login.py the auth URL is generated by appending the parameters to the auth URL,

        auth_login_return_url = auth_login_url + "?" + urlencode(auth_params)

but if the auth URL already contains ? then this generates an invalid URL.

The tool consumer https://wordpress.org/support/plugin/lti-platform/ uses an auth URL that is https://domainname.com/?lti-platform&auth so the generated URLs are invalid for this consumer.