Has there been a change to the visa system that United uses?
For my last three international flights I have not been able to complete OLCI. All flights have return itineraries with United so onward tickets available, passport and GC details are on the system. It had been a "visa" issue but no visas have been required (great advantage of an Australian passport), an onward ticket which was with United), not sure why I cannot do OLCI today but checked no visa required provided stay is not more than 90 days and return ticket is in 15 days.
Fortunately on each flight I have not been waiting on an upgrade so time of check-in is not relevant but this may not always be the case.