The above or a similar message generally indicates that something on your computer or network is intercepting and possibly monitoring your connection to the Zotero servers. The zotero.org SSL certificate should always be valid.

(If you're getting a certificate error for a proxy or WebDAV URL, that's a different issue.)

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. Make sure your system clock and time zone are set correctly.
3. 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 4.
   • (Zotero for Firefox) If Firefox displays an SSL certificate warning, there is a serious problem on your computer or network. Do not whitelist the certificate, as zotero.org's certificate should not be 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, either someone or something is intercepting your web traffic and didn't take the necessary steps to prevent the resultant warnings in Firefox or your Firefox installation or profile is corrupt. See Step 4 below for help determining whether your connection is being intercepted. You can also try reinstalling Firefox or, if that doesn't help, creating a new Firefox profile and connecting to https://sync.zotero.org from there. If you see “Nothing to see here” in the new profile, your original Firefox profile may be corrupt, and you may 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 or something may be intercepting your web traffic. Check your system for malware. Try temporarily disabling any security software you're using. If you're still having trouble, 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 or something may be intercepting your web traffic. Do not whitelist the certificate, as zotero.org's certificate should not be 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.
4. 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 intercepted, possibly due to malware or security software on your system or a proxy server on your network. The following steps may help you debug or fix the problem:
   • View the certificate information for zotero.org, which may show what is intercepting your connections.
   • Check your system for malware.
   • If you are using security software, try temporarily disabling it, or disable its SSL/TLS/HTTPS-scanning feature.
   • If you are using a laptop, try syncing from a different network.
   • If you are in an institutional environment, ask your network administrator if they have installed a “custom root certificate” in your browser. If so, you can try either using Zotero for Firefox instead or following the certificate override instructions for Zotero Standalone, but be aware that in either case your connection to Zotero is being intercepted and possibly monitored.
   • In rare circumstances, reinstalling Firefox (Zotero for Firefox) or Zotero Standalone or trying in a new Firefox profile (Zotero for Firefox) may also help.