Replication from 2.0.27 -> 2.2.4 failing

["Christopher C. Weis" <christopher-c-weis@uiowa.edu>]

When replicating from Openldap 2.0.27 to 2.2.4, things are not going 
well.  On the 2.0.27 server (the master), we're seeing a 
<slave-server>:0.rej file containing changes prefaced by:

"""
ERROR: Unknown error
"""

On the 2.2.4 (the slave), we're seeing the following log entries:

"""
Feb  9 10:02:09 <server> slapd[16870]: No structuralObjectClass for 
entry (<blah>)
Feb  9 10:02:09 <server> slapd[16870]: conn=4 op=93 RESULT tag=105 
err=80 text=no structuralObjectClass operational attribute
"""

The master's .rej file and the slave's "structuralObjectClass" errors 
seem to line up.

So, here are my questions:

-- Is replication from 2.0.x -> 2.2.x possible/realistic?
-- What the heck is the structuralObjectClass attribute, and why does it 
need to be defined when the "rules" already say each entry in the 
directory must have one and only one structuralObjectClass (at least 
this is how I understand it.)  I can't seem to find any good 
documentation, so can someone please point me toward a reference or 
explain this?

Also, we're replicating from 2.0.27 -> 2.0.27 with no problems at all 
and have been for quite some time, so I'm doubtful (but not impossible) 
it's a setup/configuration issue with replication.

Thanks.

~Chris

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