Err seems to mean it has too much water for the single cup and will stop instead of overflowing and dumping hot water all over the counter.
When it happened to me, I found that I had overfilled the system when selecting single cup. To prevent overflowing, you are only allowed to have 14oz of water in the machine when you choose the single cup. I unplugged that machine to reset it (which may or may not have been necessary), but then turn the selector knob back to carafe to dump out whatever water was still in the system. At this point, the coffee maker was entirely empty of water and the err went away.
When making a single cup, such as for a travel mug, just put the water into the travel mug and put only that much water into the machine.
-Dan