Comment 7 for bug 571057

(To be precise, if I have followed the changelog correctly, the problem will be triggered when the upgrade path looks like:
  slapd older than 2.4.17-1ubuntu3 -->
    slapd between 2.4.17-1ubuntu3 and 2.4.21-0ubuntu4 -->
       (maybe some upgrades within that range) -->
          slapd 2.4.21-0ubuntu5

The first of those upgrades would add the
   olcAccess: to * by dn.exact=cn=localroot,cn=config manage by * break
line, and then final one would add
  olcAccess: {0}to * by dn.exact=gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth manage by * break
, thus triggering the error.
)