I experienced the same problem last month returning from Europe and today trying to OLCI for flying from my native Canada to the US. After getting the standard "the web site was designed to operate this way" I spoke with a supervisor who understood the issue. She indicated that she know it used to be possible to enter a green card & OLCI. The only "work around" offered was to enter a dummy return date, which I am not prepared to do because that info is passed on to ICE. Plus others have reported that doesn't work.

When I first called DL about this from Europe, I was told that it was a known bug (and not a "feature"!). The agent at check in was also very apologetic.

Also, I do not have any foreign addresses or credit cards in my profile and I did not see where one can add a Green Card #.

IIRC I have not experienced an issue with UA.

This is very frustrating because I like being checked in before going to the airport, just in case there is a problem en route. Also, without being checked in I have no way of seeing the upgrade list.