Do you remember if you got the error while trying to solve the first OCaml puzzle or the second one?
Server logs suggest that you solved the first OCaml puzzle successfully, got a second OCaml puzzle in response, then after ~30 seconds refreshed the page, which could actually be done automatically by the browser upon network error. That refresh resulted in starting a new game.
Without more info, my only guess is that the server was too busy for the reverse proxy to fully process the request.
Unfortunately, the game isn't downloadable, and being a "Web 1.0 captcha" poses limitations regarding connection retries.
In case of an error like this (or one of the 50x errors), the best workaround depends on the browser: first, try to go back to the previous page, then either go forward (if the browser allows that) or re-submit the same diagram again. If the browser doesn't support going back to the previous page, refreshing the page may do the trick.