Diagnosis
That's the unit telling you it can't find the temperature sensor. Usually, the little sensor bulb inside the unit has come loose from its spot or the wire got pinched during a move.
How to Fix It
Unplug the unit first. You'll need to pop off the front grille. Look for a small, bean-shaped bulb on a thin wire, typically clipped near the evaporator coils (the cold coils). Gently push it back into its clip so it's snug and not dangling. Make sure the wire isn't kinked or cut. Reassemble, plug it back in, and the E1 should clear.
If That Didn't Work
The sensor itself might have failed. You'd need to test it with a multimeter for continuity, which is a bit more involved. At that point, a call to homelabs support for a replacement sensor part is your best bet.
Heads Up
When you're handling the grille and poking around inside, be careful of the sharp edges on the metal chassis and don't bend any of the delicate aluminum fins on the coils.