The issue occurs due to a self-signed certificate connection. You can read more about the nature of certificate warnings here:
1. Open main Kaspersky window - More tools - My network - Network monitor;
2. Sort the table by Port.
3. Find the port 8009 and note down the local IP address used by it (like 192.168.x.x). There can be several IP addresses.
I think I might have stumbled onto something. Yesterday I opened Google Home app on my phone and went into the device settings for both of my chromecast devices. In the privacy section I turned off "send chromecast device usage data and crash reports to Google" then rebooted the chromecast device. After finishing the same for both devices, I then rebooted my Windows 7 machine. After doing so everything seemed to be working fine. No more self signed certificate warnings, the "Cast" button on Chro