How to debug evaluation error 255 on Hydra

As a user relatively new to Nix and NixOS, I’ve found Hydra a challenge to get going with. In particular, I have a jobset which builds a flake that earlier today succeeded and is now failing to even evaluate, giving me evaluation failed with exit code 255. Evaluating the hydraJobs attr of the flake on the command line of the same machine running Hydra obviously succeeds, so I’m at a bit of a loss to know how to proceed with this.

Any tips or ideas for why evaluation could fail, and how to prevent it? I’m ultimately expecting this machine to handle building hundreds of jobsets and 10k+ packages per day, so I definitely need it to be rock solid.