Can't save modified layout in Plone 5

[hvelarde]

Plone 5.0.5 with collective.cover installed from master branch (1.2b2, unreleased).

After creating a cover object with Layout A, access the Layout tab, make some modifications and try to save them; the following error is shown:

2016-08-18 12:17:07 ERROR plone.transformchain Unexpected error whilst trying to apply transform chain
Traceback (most recent call last):
  File "/home/hvelarde/.buildout/eggs/plone.transformchain-1.2.0-py2.7.egg/plone/transformchain/transformer.py", line 49, in __call__
    newResult = handler.transformIterable(result, encoding)
  File "/home/hvelarde/.buildout/eggs/plone.protect-3.0.18-py2.7.egg/plone/protect/auto.py", line 158, in transformIterable
    if not self.check():
  File "/home/hvelarde/.buildout/eggs/plone.protect-3.0.18-py2.7.egg/plone/protect/auto.py", line 177, in check
    return self._check()
  File "/home/hvelarde/.buildout/eggs/plone.protect-3.0.18-py2.7.egg/plone/protect/auto.py", line 201, in _check
    check(self.request, manager=self.key_manager)
  File "/home/hvelarde/.buildout/eggs/plone.protect-3.0.18-py2.7.egg/plone/protect/authenticator.py", line 113, in check
    raise Forbidden('Form authenticator is invalid.')
Forbidden: Form authenticator is invalid.