Try looking at response from server particularly Content-Length.
I've been asked couple of times and also experienced failure with auto-updater on Mac OS. I've set correct URL, and tested it with curl and directly in browser and got response from server (both 200 and 204).
But somehow I kept getting
error event from auto-updater. By investigating some more, it turned out to be problem in response from server. Squirrel for Mac documentation has link to description of status code 204 No Content which contains very important line:
The 204 response MUST NOT include a message-body, and thus is always terminated by the first empty line after the header fields.
In my case, application received response with empty body, but Content-Length was 29. This actually caused auto-updater to fail, and fixing this to be 0 solved my problem.