Absolutely disgraceful that United can't get their act together on Nexus. I travelled DEN-YVR and unable to check in online and at the airport, the agent couldn't issue a boarding pass without a passport. If NEXUS is good enough for CBP, it should be ok for United. Their computer system doesn't seem to accept NEXUS as a valid travel document.

My travel colleague had the same problem and ended up with a BP that didn't have 'Pre-Check' printed on it so got turned around at security for being in the wrong lane, despite having NEXUS and basically being 'qualified' for that line. At least my BP had Pre-Check on it and lucky we both had our passports with us.

This should be a really simple fix for United, I don't know why they can't do it, its apparent this is not a one off incident from this thread.