The above message sync error generally indicates a problem with either your Firefox installation (if you're using Zotero for Firefox) or your network. The zotero.org SSL certificate should always be valid.

Try the following steps to debug the problem:

1. Restart Firefox (Zotero for Firefox), Zotero Standalone, and/or your computer and try syncing again. This error can be caused be intermittent network issues.
2. If you get the same error after restarting, load https://sync.zotero.org (with 'https', not 'http') in your browser (e.g., Firefox, Chrome, Safari).
   • If you see the message “Nothing to see here”, continue to Step 3.
   • (Zotero for Firefox) If Firefox displays an SSL certificate warning, there is a serious problem with either your Firefox installation or your network. Do not whitelist the certificate, as zotero.org's certificate is not invalid and you will no longer have any guarantee that your connection to zotero.org is secure. To see if the problem is on your computer or your network, try loading https://sync.zotero.org in a different browser.
     1. If you see “Nothing to see here” in the other browser, your original Firefox installation or profile may be corrupt. Try reinstalling Firefox. If that doesn't help, create a new Firefox profile and connect to https://sync.zotero.org from there. If you see “Nothing to see here” in the new profile, your original Firefox profile is corrupt, and you will need to either repair it or copy your data to the new profile, including your Zotero data.
     2. If you see the certificate warning in the other browser, someone may be intercepting your web traffic. Talk to your network administrator or ISP. If they tell you to whitelist the certificate, be aware that your connection to zotero.org will no longer be secure.
   • (Zotero Standalone) If your browser displays an SSL certificate warning, someone may be intercepting your web traffic. Do not whitelist the certificate, as zotero.org's certificate is not invalid and you will no longer have any guarantee that your connection to zotero.org is secure. (Whitelisting the certificate in your browser will also have no effect on Zotero Standalone.) Talk to your network administrator or ISP. If they tell you to whitelist the certificate, be aware that your connection to zotero.org will no longer be secure.
3. If you see the message “Nothing to see here” after loading https://sync.zotero.org in your browser but continue to get SSL certificate errors in Zotero, your sync requests may be getting interrupted, possibly due to security software or a proxy server on your network. If you are using security software, try disabling it. If you are using a laptop, try syncing from a different network. In rare circumstances, reinstalling Firefox (Zotero for Firefox) or Zotero Standalone or trying in a new Firefox profile (Zotero for Firefox) may also help.