While it's hard to diagnose with certainty what is causing the dieback, typical ailments for a variety of boxwood cultivars include Volutella Blight and winter injury (often working in concert...the cold damage occurs first, then the fungal blight takes advantage of the weakened areas). You can explore our boxwood diagnostic page for more information and typical pests/diseases of boxwood. The stem close-up looks normal; that knobby, warty look for the bark is typical of mature boxwood wood.

All you can do for now, regardless of the precise cause of dieback, is to prune out / cut down all declining or dead growth, since that branch will not regrow. You can cut back healthy wood as well if you would like the plant to look more balanced while it recovers. If the roots remain healthy (as generally is the case for Volutella), new growth may arise from the base of the plant after a drastic pruning, as illustrated in the attached photo of a mature boxwood shrub that was pruned back to within about a foot of the ground. Worst-case, if that is either an unappealing solution or doesn't work well, the shrub will need to be replaced.

Miri