Error 27/28 in UPS Online UPS1000/2000

In the Online UPS some errors 27 and 28 may appear on the screen:

These UPS identify the phase and neutral of the current connector.

So, if when connecting to the plug, the phase, and neutral of the UPS connector do not match those of the plug, then errors 27 and 28 will appear.

The solution is to turn the connector in the plug so that the errors match and disappear: