ldap_modify: Other (e.g., implementation specific) error (80), <olcAccess> handler exited with 1

This is because the line continuation in the ldif file removes the first space and the attribute is considered to be: {0}to *by dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" writeby * none (which is invalid)

So, put an extra space in front of the line continuations:

dn: olcDatabase={2}hdb,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to *
  by dn.base="gidNumber=0+uidNumber=0,cn=peercred,cn=external,cn=auth" write
  by * none