i think it maybe the newish "malware/ phishing protection" in chrome, if calibre is not explicitly whitelisted:
I found this & am about to test toggling the setting:
In recent update of Google Chrome internet browser (in stable channel), there is a new security/privacy option:
In addition to checking a list of known bad files, Chrome also does checks on executable files (like ".exe" and ".msi" files). If the executable doesn't match a whitelist, Chrome checks with Google for more information,
such as whether the website you're accessing hosts a high number of malicious downloads.
According to more detail link
If a file isn’t from a known source, Chrome sends the URL and IP of the host and other meta data, such as the file’s hash and binary size, to Google.
UPDATE : NO - it's not that setting ...