Hi-
Quick follow up: Zimbra got in touch with me off-list and asked me to open a case and provide some debug-level information. I've done so, I'll report back here once we've got any sort of resolution. Our case # is 00053746 just in case you might need to reference it in similar discussions with support.
-- dNb
P.S. Very happy with Zimbra's reaching out to me first to help get this resolved.