Region

Remove all power from the unit. Let it sit a few minuets. Plug it back into the wall outlet. This should resolve the problem.
Put the dvd into the unit that you were having problems with, and try and play it. If it works - everything is ok. If not - try repeating the process 2 to 3 more times. What is going on here is the Unit automatically assigned a regional list in its memory. Sometimes this happens during an AC power surge (noise) on the line. This can corrupt the macienes internal memory. Bypulling the plug you allow the internal circuitry (typically some capacitors) to discharge. After this the unit will re-set & the error will go away.