Collective.revisionmanager permissions

I have a Plone 5 site where I want to purge revisions. When going to the collective.revisionmanager control panel ( @@revisions-controlpanel ) I get

Error You are not allowed to manage the Zope server.

I tried logging in as Zope (admin) user and Plone (site) admin without luck.

Any suggestions?

UPDATE: I hacked around it by (temporarily) adding 'view management screens' permission to 'Authenticated' in Zope root ( Manager, Owner etc already had permsissions but that did not 'work')