I upgraded a Plone 5.1b3 to 5.1rc1. Now when I log in as a user who does not have (zope) manager permissions, the toolbar does not show up. This effect does not appear in a freshly generated Plone 5.1rc1 instance.
first debugging on that issue together with @vincero shows that the Permission 'Show Toolbar' is present in the Security Tab (ZMI) but not that the actual default (Aquire false and Autheticated true) is set.
I guess there is an upgrade step for the rolemap is missing.
We should open a ticket for that.
It that's the case then it is a blocker for rc1. Please open a ticket in CMFPlone asap.