Calling the WHO pediatrics growth charts gives the traceback:

Traceback (most recent call last):
  File "/trytond/protocols/jsonrpc.py", line 162, in _marshaled_dispatch
    response['result'] = dispatch_method(method, params)
  File "/trytond/protocols/jsonrpc.py", line 191, in _dispatch
    res = dispatch(*args)
  File "/trytond/protocols/dispatcher.py", line 162, in dispatch
    result = rpc.result(meth(*c_args, **c_kwargs))
  File "/trytond/wizard/wizard.py", line 275, in execute
    return wizard._execute(state_name)
  File "/trytond/wizard/wizard.py", line 285, in _execute
    view = state.get_view(self, state_name)
  File "/trytond/wizard/wizard.py", line 83, in get_view
    return Model_.fields_view_get(view_id=view_id, view_type='form')
  File "/trytond/model/modelview.py", line 340, in fields_view_get
    tree = etree.fromstring(result['arch'], parser)
  File "src/lxml/lxml.etree.pyx", line 3213, in lxml.etree.fromstring
(src/lxml/lxml.etree.c:77697)
  File "src/lxml/parser.pxi", line 1818, in lxml.etree._parseMemoryDocument
(src/lxml/lxml.etree.c:116475)
ValueError: can only parse strings