You mean ADI-2 DAC FS, not ADI-2 FS (without LCD display), which is another device in existence.
Either unplug the ‘phones and/or IEM, or set “Mute Line” to “OFF”, “Toggle Ph/Line” or “Toggle Plugged”, depending what you prefer.
From the manual page 27:
14.1.3 Phones / DSD / Power
The subpage Phones / DSD / Power has the following entries:
Mute Line
OFF, vs. Phones, Toggle Ph/Line. Toggle Plugged. The default, vs. Phones, activates mute of Line Outs when inserting a phones or IEM plug. Alternatively Toggle activates the ability to manually switch between 'front' and 'rear'. It allows to have the Phones/IEM connected all the time. To change push the VOLUME knob for half a second. Via Remap Function Keys this function can also be controlled by one of the four function keys or the remote control. Toggle Plugged additionally activates the switching between Phones and IEM, automatically detected by the sensor contacts of the two outputs. Which also works via Remote and Function Keys, if Toggle Ph/Line has been activated there.