If you've already spent hours on the phone with Apple and gotten nowhere, there's not likely going to be a better solution offered here.
The first try would be the combo update, and that didn't resolve this.
The next is what Apple recommended. Haul off a backup copy (or two) to external storage, wipe the disk, and reload OS X Server. That's a straightforward sequence, and the OS X Server installation can migrate in the old settings (migrated in from an external disk), and that will hopefully resolve whatever corruption is lurking here.
I've done various of these reinstalls with 10.6 servers, and they've cleaned up some odd errors.
You are saying that Mail doesn't launch?
Forget that this is a server. Troubleshoot it the same as-if it were a workstation.
What have you tried so far?
Some typical things to try:
- login with a different user.
If that user doesn't have an issue, then the problem is within the original user's home.
- Assuming u have a backup.. try erasing everything to do with mail
Delete or rename ~/Library/Mail
and delete the mail prefs.
- What do your logs reveal?