Stack Overflow: “I got into a similar state just today. It was very odd: I could attach any XIB (new or existing) to any already-existing ViewController class in the project, but I could not create a new ViewController and have it attach properly to any XIB. The “assistant” functionality didn’t work, nor did the IB-to-headerfile-connection functionality.”

This has become an everyday annoyance so I’m documenting the link to the “fix.”