EWR-LAS
My best guess of what happened is this. Recently TK removed the NYC option from their website. Now you have to individually search flights for JFK and EWR. It used to be that the only way to find flights from LGA was by searching for NYC flights, so when they removed that option LGA was blacked out. They then added LGA as a search option, so it is possible to find flights from there, but somewhere in this mess something in their system broke and has blocked all availability from EWR. It’s not just United, even flights from EWR to Europe on other airlines are not searchable. I booked a Star Alliance flight with them last week (that did not touch EWR) and was actually surprised by how competent the agent was. But it appears that for now EWR is just a dead spot in their system.
(Note: I have no background in computer science and may just be spewing gibberish, but I can’t believe it’s a coincidence that EWR stopped working just as they removed the NYC search option from their website)