That is the exact same lnk I use to check my email from the Xfinity Home page.
I am not using Bitdefender - using Norton! I have no reports of it not being safe.
Perhaps a false positive from Bitdefender or something from the browser you are using?
It is very odd. I use google as browser. I have checked the starting page under Google settings. The xfinity website works okay...I just keep getting notifications saying the website is not safe.