This error means that we can't find the certificate on your machine anymore. It would have been registered when you configured Agent originally, but now it's not there anymore.
If you are an expert user and have been playing around with mmc.exe, did you by any chance delete the certificate from Current User\Trusted People? If so, that's what the problem is.
You can try to fix the problem if you still have the certificate file (PFX or P12):
- Go to the Settings section and click the edit button next to Credentials
- Browse to your certificate file and specify a password
- Click Sign In and that will restart Agent
If it's still not working, please click on the link that says "Report this error", which will open an email to firstname.lastname@example.org with the error details. If you don't have an email application configured on that machine, you can click the Copy to clipboard link to get the error message, then send it to us from another machine.