The plugins api on wp.org is using an antique version of the Sphinx API client library, 0.9.8.1. I don’t know what version of the server software is running, there doesn’t seem to be a way to query that.
I’d like to upgrade it if it’s not already running a recent version. I tested the current stable PHP client, 2.0.1, and it seemed to work fine. Presumably that means it’s safe to deploy with the new client library first and update the server later. Or perhaps it’s already running a newer server.
What version of the Sphinx server is running currently?