The printer informed me that the front cover of the tool head had fallen off. It seemed to be fine, but the tool head was pressed up against the front of the printer. I wanted to just pull the cover and re-seat it, but this was impossible, because of the location/position of the tool head. And there was nothing I could do to move the tool head, because I was stuck in the dialog about the front cover having fallen off. I had to kill the print job (rather than re-seat the cover and proceed with the print). I then removed the front cover and replaced it, and have had no further problems. The warning was spurious.
Constructive suggestion: when the X1C emits that warning, position the tool head in the middle of X and Y so that the tool head can be inspected and fiddled with, and the print can continue without having to re-start the print.