Similarly to users and groups, each userGap object composing the user calendars is associated to a securityScope object via the 'userGap.scopeRef' attribute. This securityScope defines the access permissions on the userGap objects.

The 'New' function of the 'User calendars' page creates the userGap objects in the 'HR' (for 'Human Resources') securityScope - that MUST exist in the system. This security scope grants editing permissions to the members of the 'Administrators' and 'HR' groups, and only view permissions to members of 'Everyone', so you must be either a System Administrator or member of the 'HR' group to be able to edit user calendars.