FYI, I am experiencing a similar/the_same problem
Also trying to migrate Microsoft Server 2003 Small Business Server (aka SBS 2003) with exchange to Zimbra 7, and the migration wizard also fails.
PST import is going to be a real pain, because the mailboxes are 80,000 emails large.
(I have a zimbra support case open: SF: Case 00068764
8.x NE + OSS KVM guests and Physical servers Ubuntu 12.04, RHEL 5 + 6 .
on Dell server hardware.
Previously: ZCS OSS and NE 7, 6, 5, 4, 3 variously on physical and virtual