OK,
I think I figured something out. This was a very difficult install, getting everything just right, and I spent weeks (2-3) troubleshooting. I do lots of FOSS installations and I found this one to be one of the toughest. Once I got everything working, It appeared to stop working with this bug, being able to access the shared briefcase but not being able to download.
I emailed everyone in our test group that it looked as if it was broken, and I recommended that we share a generic account, as we were using this just to share documents online anyhow. Nobody needed the mail features.
At some later point I logged in and it seemed to be working again. As fortune would have it, nobody's using this system for several weeks now, so my recollections are a bit fuzzy, but it seems that I was premature in telling my people that the downloads weren't working.
I was testing from my local network
https://192.168.1.225 , and they were coming in from https://<domain I registered just for testing this>.com . At some point I figured out that it was working all along (as I recall - didn't painstakingly test this) when I accessed via the domain name, but briefcase downloads were broken when came in with a localnet IP address.
So my apologies to the Zimbra folks for my error. I WAS aware that this thing is extremely DNS sensitive, and I think I even had a hosts file entry on one or more of my client machines.
-mz