Edge worked for me for a while but then gave the "session expired" message; then after a few minutes of me doing nothing, Edge allowed me to search for flights again. I wonder if there's a time limit of "you can do no more than X searches in Y minutes" using Edge on United.com, very possibly related (as someone mentioned earlier in the thread) to UA not wanting bots scraping fares from their website.

Interestingly, Chrome/Brave on MacOS worked fine for me for award fare searches, but Chrome/Brave on Windows immediately gives the "session expired" error. AFAIK, bots are rarely (or never?) installed on MacOS.

I complained too but I doubt this will be fixed unless someone high up at UA notices a drop in sales from the website and wants an investigation. My previous complaints about UA IT not working correctly were ignored. The problem IMO is that UA didn't do enough testing when making changes to their website. It's not possible to test every combination of operating system version, browser version and browser add-ons but it is possible to test common combinations: however, UA seem not to have done so.