I've been having issues lately as well, but I think most of them were self inflicted.
About 3 weeks ago, I changed my United Password and forgot to change it on the MileagePlus X App. I had FaceID turned on and my iPhone kept trying to log me in with my old password. I got the generic "Transaction Declined" message and had to wait the 2 weeks for it to unlock.
I tried it again last Wednesday and it worked great. On Saturday evening, I got a different Transaction Declined" message, telling me to check my billing address in my mileageplus account. I checked them and they were right. So I tried a few different cards, and tried my wife's account. After a bunch of failed attempts, I got the normal "Transaction Declined" message and now have to wait the two weeks again. 🤦♂️