I actually had this issue yesterday. I had to connect using the DAC after starting SQL with the -f option. When trying to connect with just a trusted connection or the "sa" login, I would get login failures. DAC went right in. I haven't seen anything in the documentation about needing DAC but if someone runs into this, don't forget it's available.
"There are two ways of constructing a software design. One way is to make it so simple that there are obviously no deficiencies. And the other way is to make it so complicated that there are no obvious deficiencies."
"If you think it's expensive to hire a professional to do the job, wait until you hire an amateur." Red Adair.