Invalid certificate warning or "cannot establish a secure connection"
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Often this due to a content filter (for example a router's parental controls) blocking the site and showing a security-style message instead of a clear "blocked" notice. Check that first — see "What if I have an internet content filter or firewall?".
If it isn't a filter and you're on a Mac, iPhone, or iPad, the device may be caching bad security-certificate data:
- Clear your browser cache and reload.
- Make sure the device's date, time, and time zone are set to update automatically — certificate checks require an accurate clock.
- Restart the browser (and the device) and try again.
This is a device setting rather than a Blink Session issue; Apple's support site has detailed steps for iPhone and iPad if it persists.