This happened to me about a week ago. It was a problem that was tied to my account and I had to call web support to get them to clear it. Changing browsers or computers won’t work, since it was clearly an account issue. Waiting it out didn’t work. The app still worked but the web site didn’t work in certain areas, such as searching for flights .

my symptoms were exactly the same as described by the OP.

They never explained it, but they fixed it after waiting about 24 hours. Call a United agent and ask to be transferred to Web support. I think I had to call twice before they finally cleared it.