This was a real bug in the rc1 release. It triggered when the request payload was large: users whose ticket payloads carried extra data like thumbnail references got a Faraday::Multipart::CompositeReadIO instance passed to the upload middleware's set_file, which called key? on it and crashed. The workaround is to use _id suffixed attributes (send ids only, not whole objects) when you have existing records, since the gem is not exactly like ActiveRecord. The actual fix was pushed in the rc2 build, which the maintainer confirmed resolves it. So update to a version containing the rc2 fix, or use the _ids workaround if you are pinned to rc1.
Source: https://github.com/zendesk/zendeskapiclient_rb/issues/513