The above or a similar message generally indicates that something on your computer or network is intercepting and possibly monitoring your connection to the internet.

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

Try the following steps to debug the problem:

1. Restart Zotero and/or your computer and try syncing again. This error can be caused by intermittent network issues.
2. Make sure your system clock and time zone are set correctly.
3. If you get the same error after restarting, Zotero's network connection may be getting intercepted, possibly due to a proxy server on your network or security software or malware on your computer. View the certificate information for zotero.org in your browser, which may show you what is intercepting your connections.
   1. If your browser shows zotero.org as verified by “GeoTrust Inc.”, “RapidSSL”, or “Amazon”, Zotero's network connection may be configured differently from your browser's.
   2. If your browser shows zotero.org as verified by something else and you recognize the listed entity (e.g., the name of security software or your institution), take appropriate action. If you don't recognize it, try the following steps:
      • Check your system for malware.
      • If you're using security software, try temporarily disabling it, or disable its SSL/TLS/HTTPS-scanning feature.
      • If you're using a laptop, try syncing from a different network.
      • If you're in an institutional environment, ask your network administrator if they have installed a “custom root certificate” in your browser. If so, you can try following the certificate override instructions for Zotero, but be aware that your connection to Zotero is being intercepted and possibly monitored.
      • In rare circumstances, reinstalling Zotero may help.