Mine doesn't have leaves that are so serrated, but this looks like elderberry to me.
Trace the wilting branch down to the trunk. Older Elderberry trunk die off each year, and borers get into the deadwood. If they bored down to where the new shoots/branches are growing, that would kill the branch.
Try cutting the trunk where the wilty branch connects and see if there is a hole in the middle of the pithy white core.
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