Actually serguimartin that index error is:
Bug 42599 – Message causes NPE on ReIndex - indexing blocked
You can see some more discussion on that in this thread:
Problems with mail search since upgrade to 6.0.1
As for the problem described in this thread... here is a wild guess but what do the olcAccess lines in
Code:
/opt/zimbra/data/ldap/config/cn\=config/olcDatabase\=\{2\}hdb.ldif look like?