Grafana's secret key (services.grafana.settings.security.secret_key) doesn't have a default value anymore. Please generate your own and use a file-provider on this option! See also https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/#secret_key for more information. See https://grafana.com/docs/grafana/latest/setup-grafana/configure-security/configure-database-encryption/#re-encrypt-secrets on how to re-encrypt.
As stated in the NixOS changelog for 26.05, there's no official way to rotate.
Either hard-code the old key ("SW2YcwTIb9zpOOhoPsMm") if your setup doesn't have any secrets in the DB that need special protection or perform a rotation with a 3rd-party tool (d9dc788902).