Hi Fstop4.5,
Thanks for reaching out and welcome to Apple Support Communities! We understand you're currently unable to access a Capital One website using Safari, and we'd like to help. To clarify, is your Mac a personal device, or is it company-owned? Also, is there a chance that it may have a profile or VPN network installed that could be blocking the website? If not, are any other websites experiencing the same problem? These details will give us a good place to start from.
Also, you may try restarting the Mac in safe mode using the steps here: How to use safe mode on your Mac
Starting up in safe mode will help us determine whether the problem is happening with macOS, or if there's a possible third-party software conflict. We look forward to learning the results!
Have a great day.