I'm having the same issue including not being able to log into my Cox account. I use Google Chrome and have been getting that error for a couple of weeks now. I tried clearing cache, uninstalling chrome and removing all data but the error persists.
I finally tried Firefox and it works fine. The weird thing is that the opposite is true on my phone. Firefox won't work but Google Chrome does.
As I was typing this, I cleared cache and data on the Firefox app on my phone and now it seems to work.
So far, everything I've tried to get Google Chrome to log into Cox Webmail has failed. Cleared cache, data, cookies and even tried uninstalling Chrome all together. There must be some corrupt file chrome is using that does not go away with a simple uninstall.