That's an easy one: the jailbreak includes the developer certificates. But they are normally lost when the firmware is upgraded, so if you do have some hacks which depend on them, you need to either reinstall them, or re-jailbreak. All of that is stated in the JB thread, and even the README.
That said, this (Screensavers) hack does not depend on the dev certs, but is known to cause issues after a FW update. I'm not its maintainer -- in fact, I never installed it because I don't care about screensavers, so I don't know why that is. But as far as I'm aware, it started working again for most people who simply uninstalled, then reinstalled it. So maybe try that (including the "uninstall" step).