I have a Mac Mini 2012 model, and an HP ZR30w display. They worked fairly well together, using a Display Link -> Display Link Mini cable plugged into the Thunderbolt connection.
Even while this combo worked I had wake-up issues, in that the screen remained off until I unplugged and replugged the DisplayLink cable.
Then all of a sudden I could no longer log in: The Mac booted until I could click my user, and after entering the password I saw the eternally spinning color wheel. When I used another display via DVI, everything worked as before.
Now, a few days later, the Mac crashes almost immediately. Sometimes I get until the logon screen, but then the Mac reboots spontaneously. As before, a DVI connected display works fine.
So my question is: How can this be? The display can obviously be detected and used, otherwise I couldn't see the logon screen. What exactly is causing the later reboot?
For those that are interested: The culprit was a USB card reader plugged into an USB 3 hub. When this device is connected, I can't login as I described in my first port.
Most likely the later plug/unplug damaged the cable, so that I got the further issues when the display didn't even light up.
Indirectly, the posting about running the hardware test pointed me into the right direction: Before the test, one is told to unplug all USB devices. I did this, and everything was fine. Today, I finally replugged everything and it stopped working. A few trial and errors later I knew which device was to blame.
After boot I can use the card reader, though, so it is not a big issue for me.