In my case the issue was caused by the behavior of the /oauth2/register endpoint implemented in the django-oauth-toolkit-dcr library.
Specifically, when a DCR request includes any value other than "token_endpoint_auth_method": "none", the library defaults to returning "token_endpoint_auth_method": "client_secret_basic" in the response, even if the client explicitly requested "client_secret_post".
This mismatch triggers the error from OpenAI:
"MCP server <url> does not support client_secret_post token endpoint auth method"
After applying a small patch to ensure the endpoint returns the requested token_endpoint_auth_method, everything worked as expected.