https://bugs.openldap.org/show_bug.cgi?id=9915
--- Comment #1 from Quanah Gibson-Mount quanah@openldap.org --- This in part appears to be an issue where the configuration is replicated from a config node, as the olcUniqueURI doesn't appear to be parsed for validity in that scenario.
Trying to reload the config DB with serialize quoted as per the man page results in:
slapadd: could not add entry dn="olcOverlay={2}unique,olcDatabase={2}mdb,cn=config" (line=2582): <"serialize ldap:///?uid?sub?"> invalid ldap urilist