We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
"[Proxy or WebDAV URL] uses an invalid security certificate. The certificate is not trusted because […]"
If a Zotero error report shows an error similar to the above for your institution's proxy or WebDAV server, the server is likely misconfigured and will need to be fixed by your IT department. Point them to this page along with the URL from the error report.
If you're getting a certificate error for a zotero.org or amazonaws.com URL — for example, while syncing — that's a different issue.
This error generally occurs because the proxy or WebDAV server isn't serving the necessary “intermediate certificate” for secure connections, and Zotero (like Firefox, on which it is based) won't download it on its own. Without an intermediate certificate, it's impossible to determine whether the connection (which might include login details) is secure, and the connection fails.
To verify that this is the case, submit the URL from the error report to https://www.ssllabs.com/ssltest/ and view the results. If you see “Chain issues: Incomplete” in orange under “Additional Certificates (if supplied)”, you're experiencing this issue. The report will then also say “Extra download” (instead of “Sent by server” or “In trust store”) for one or more certificates listed under “Certification Paths”.
If you're able to load the same URL in a browser (making sure to use “https://”), either the browser is downloading intermediate certificates automatically or you previously loaded another site of your institution's (or another institution's) that included the intermediate certificate, which the browser cached and is using even on sites that don't serve it properly. Generally speaking, though, sites should always serve their intermediate certificates and are broken if they don't. In the case of Firefox, if you create a new Firefox profile, you should get a certificate error trying to load the same URL, which is essentially the situation Zotero is in.