Things have improved since my last post, despite the fact that I managed to kill Thunderbird along the way and lost a good chunk of today figuring out to restore it. Luckily I can read French, or I may never have found the solution to the elusive -203 problem.
Next, reinstall all your extensions. I use these 15 extensions.
Here's how the data flows, drawn with JBoss Tools' jBPM Graphical Process Designer:
I don't yet have a solution for Zimbra contacts & calendar being accessible on the Blackberry, short of manually duplicating entries from one repo to the other. Still, 5 out of 7 data sources synched is pretty decent.