Hi WTK. BT is British Telecoms in the United Kingdom, they supply landline telephone and internet service provider services such as broadband. If you have something popping up when you are not active on the system and don't have an email or a browser window open connecting to the outside world and the mentioned SSL I'd suggest updating your Malwarebytes then doing a scan as it could be something on your system not just on a website page, I'm technical but not an expert, posting in the general MB forum might help more with that.
Thanks CICO. I wondered if they were aware their domain is blocked by a major software and if they knew of any IP address issues, I tested different websites and each loaded okay but had a popup about riskware from the same domain and with different IP ranges. Help guide says..
"I received a notification on a safe site, why? If a notification is presented on a safe site, and the site loads, it is likely the site was loading content that is hosted on an IP known for malicious activity. In this case, the site itself will be displayed perfectly fine, with the malicious content being blocked. If however, the site does not load, it is likely the site is also hosted on the same malicious IP address. It is also entirely possible that the site in question, shares it's IP address with other malicious domains." (link).
If several users only just started seeing the problem possibly a software definitions update went out today. I wouldn't think a large company that provide SSL and privacy would have unsafe IP ranges for SSL. I assume it's likely to be a false positive that gets removed in an upcoming software definitions update. Will wait for an update.