I'm using my UA miles to upgrade a family trip to Europe. We are under 3 different reservations.

This morning, I noticed that the upgrade option went from waitlist to available.

I did my own reservation first. No matter which of my 6 credit cards I tried, the UA website simply wouldn't accept them. All it said was that the credit card could not be processed. My banks said there were no authorization attempts at all. I ordered something random off Amazon(amount much higher than the co-pays) and they went through immediately.

I then asked my family members and tried their credit cards on their reservations. Also got the same message. In all, we tried 11 different credit cards.

We ended up using debit cards. Three different debit credit cards went though within seconds. So we are now upgraded on all our outbound flights to Europe.

I have tried it on different computers and phones, as well as different PC and mobile browsers. Was there a problem with UA's credit card processor? (although that wouldn't explain why debit cards from the same banks went through with no issues?)