I have a black lace a year or so old. There is new growth on the branches but the old growth is turning brown. Searched for afids and borrowers but no sign. It sits in a pot and water on a regular basis with letting to dry every once in a while so as not to drown the roots. I'm not sure if I should prune this plant down as it is mid summer.
Elderberry is a sprawling, large shrub with very active roots. Your pot looks too small for it, IMO. Chances are it is rootbound if you didn't "pot up" significantly this spring. It does like moist soil but sitting in water over 30 minutes isn't the answer. Water will displace oxygen, something the roots need. Leaves also fall is they aren't getting some sun or if gets too much hot afternoon sun and is heat stressed. This is a hardy plant, though so I believe it will recover.