Something to consider, if you are running through a web filter, such as the Sophos UTM or most proxies, it is possible that the traffic is being intercepted for filtering and then re-sent to your PC. This would result in a different identification certificate and you would see a pop up with exactly this error. Installing the certificate will not stop the pop ups since the device sending it to you is not the device expected.
This can also happen if the network location installed their certificate improperly, or if your certificate store is out of date.
If this is your computer, you can fix it by updating your computer with Microsoft's windows update or installing the certificate as trusted.
If this is not your computer and there is no proxy or web management security appliance on the network then you may need to review your system for malware as it is likely you are being redirected. There are many possible ways to do this, so it is best to use software such as malwarebytes to assist in cleaning and fixing your system, but doing so manually is possible even if "painful."
If there is a chance of a proxy or a web appliance being used than the administrator of that appliance would need to adjust their certificates to reflect their system properly, or change the filtering to transparent mode (which means they would not be able to decrypt SSL traffic of any sort and must rely on URL filtering for SSL traffic instead), or the proxy is incorrectly configured.
In my opinion it is more likely that you have malware on your system redirecting your traffic, as administrators of large organizations would have a handle on certificate management. But I notice that your displayed certificate is for *.btrll.com a documented site with this error. Nortan/Symantic users report getting this error constantly on Weather Underground, which uses a different URL than the certificate is for. It is possible that the incorrect certificate was loaded by the administrator, but much more likely that people who visit the website they want are being redirected against their will...to which this pop up is notifying you of the change.
Resolution: Run antivirus and malware scans on your computer, avoid using the website that is causing this problem (sometimes it is a program that accesses the network and not a website, keep that in mind) until the problem is resolved, and lastly...contact the administrator for the problem site or program and let them know of the problem. It is best to do it in that order.