To reproduce, in a Python notebook:

def foo():
  import JupyROOT  # or import ROOT
  print("foo")

import multiprocessing
p = multiprocessing.Process(target=foo)
p.start()
p.join() # hangs here

If ROOT is imported in the main kernel process of Jupyter, the issue does not happen. This might have to do with initializing JupyROOT in a process that is not the main kernel process.

EDIT: reported by @daritter