Just thought I'd bump this up since I'm trying to book a multi city SEA-PHX-LAS-PHX-SEA since it's cheapest on US air vs. $100 more for UA metal and less miles. I get the same error. The departing flights book into K and the return legs 2 days later book into U. They all show 9 availability (well one of the flights I can select a UA codeshare or US flight #...the codeshare shows K0 and the US flight shows K9). Should I just call web support to book it?