DIAGNOSIS The three-beep error is the E7's anti-collision system kicking in. It thinks the desk is hitting an obstruction, even if you don't see anything.
FIX The desk needs to be recalibrated. Here's how: 1) Clear everything off the desktop. I mean everything—monitors, arms, pencil cups. 2) Press and hold the 'M' button and the down arrow simultaneously for about 5 seconds until the display flashes. 3) The desk will now descend. Let it go all the way down until it stops automatically. 4) It will then rise all the way to its max height and stop. That's it. The desk now knows its true upper and lower limits. Test it by using the up and down arrows normally.
IF THAT DIDN'T WORK If it still beeps and stops, there is likely a genuine physical bind. Get on your hands and knees and check that the feet are level on the floor and that no cables (especially the one between the two legs) are pinched or tangled in the lifting columns. Also ensure the desktop itself is perfectly square on the frame and all bolts are tight.
HEADS UP This often happens after a move or if you initially assembled the desk on an uneven surface. Doing the recalibration on a perfectly level floor is crucial for it to work right.