We've seen a few of these in the last week and it appears to be an out of date security certificate.
We have a patch in the works but if you need it now you can go to this site: https://www.amazontrust.com/repository/
Under the heading "Root CAs" click the DER from the first link under "Self Signed Certificate"
Download, save and install the certificate.
Close and reopen Chief and try the download again. If you continue to have trouble after that please reach out to Technical Support so we can look closer at what you are experiencing.
CarrieS's post in Catalogs won't download from Library Browser was marked as the answer