This was posted briefly on the UA homepage yesterday:
We’re aware of a temporary issue affecting our website’s performance. We're sorry for the inconvenience and recommend visiting later
Apparently that issue was affecting the purchasing of upgrades. I was on a seat map page and for an upcoming transatlantic flight and it offered $125 to select a Polaris seat, but on the reservations upgrade page it wanted $2528. I thought it was an April Fools joke. The issue was rectified before I had a chance to try and buy it... Maybe related to what TheOnlyWaytoFly describes.