cubicweb #5753280 SESSION_MANAGER can be None [validation pending]

On an instance behind pyramid, the /siteinfo path yields the following trace:

activites (cubicweb.web) ERROR: an exception occurred while calling js_view([]): 'NoneType' object has no attribute 'current_sessions'
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/cubicweb/web/views/ajaxcontroller.py", line 143, in publish
    result = func(*args)
  File "/usr/lib/python2.7/dist-packages/cubicweb/web/views/ajaxcontroller.py", line 284, in __call__
    return self.serialize(implementation(self, *args, **kwargs))
  File "/usr/lib/python2.7/dist-packages/cubicweb/web/views/ajaxcontroller.py", line 376, in view
    return self._call_view(viewobj, paginate=req.form.pop('paginate', False))
  File "/usr/lib/python2.7/dist-packages/cubicweb/web/views/ajaxcontroller.py", line 246, in _call_view
    view.render(**kwargs)
  File "/usr/lib/python2.7/dist-packages/cubicweb/view.py", line 136, in render
    view_func(**context)
  File "/usr/lib/python2.7/dist-packages/cubicweb/web/views/debug.py", line 137, in call
    sessions = SESSION_MANAGER.current_sessions()
AttributeError: 'NoneType' object has no attribute 'current_sessions'