I've discovered that if you have united.com open in multiple tabs on your browser (in my case, Mac/Safari), the site gets very confused and it becomes quite dysfunctional. Symptoms include the page loading and reloading multiple times in a row and overall strange behavior. This can happen if I've opened united.com earlier in the day, forgotten to close that tab, and then open a new tab to united.com. Or, if I'm trying to work in two tabs - maybe viewing a reservation in one tab and booking in another tab.

Closing other united.com tabs, or restarting the browser and keeping your work on united.com within a single tab, generally fixes the problem.

I used to have problems all the time with the web site, since I tend to be someone who has dozens of browser tabs open at a time, but things have been much smoother since discovering this issue. And, when I have problems now, I go through my browser tabs and always find one or more that are sitting at united.com.