The insufficient connection time error is a common default. You will almost always see this warning with multi city because it starts showing the earliest flight on connections which may not even be possible. IF it is showing alternate flights, select the appropriate legal flight. IF it is not showing any flights, try selecting timings on your input that you know would be legal. Sometimes I get it to work by just searching again and changing nothing.
This is an example of v crude IT at UA. I know they must be working with some legacy systems, but from a consumer perspective, this is POOR. Good luck....