Hi Alex and Damien,

I tried to load the LDIF provided by Damien with raw LDAP tools and made 
an observation which may help, (or may not, I am still a Schema 
Subsystem rookie).

Loading this attribute type (as provided by the LDIF) fails with "The 
attribute type syntax is invalid"

dn: m-oid=1.3.6.1.4.1.13769.3.7,ou=attributeTypes,cn=moz,ou=schema
objectClass: metaTop
objectClass: metaAttributeType
objectClass: top
m-equality: caseIgnoreMatch
m-name: mozillaHomeUrl
m-oid: 1.3.6.1.4.1.13769.3.7
m-singlevalue: true
m-syntax: 1.3.6.1.4.1.1466.115.121.1.15

But this one works

dn: m-oid=1.3.6.1.4.1.13769.3.7,ou=attributeTypes,cn=moz,ou=schema
objectClass: metaTop
objectClass: metaAttributeType
objectClass: top
m-equality: caseIgnoreMatch
m-name: mozillaHomeUrl
m-oid: 1.3.6.1.4.1.13769.3.7
m-singlevalue: TRUE
m-syntax: 1.3.6.1.4.1.1466.115.121.1.15

Please note the only difference: "true" vs. "TRUE".
Perhaps it has something to do with enabling syntax checking of 
attribute types?

Greetings from Wiesbaden,
     Stefan Zoerner (szoerner)