1. Synced a bunch of ZCO 5.0.1x clients to (a fork of) production 5.0.19 server.
  2. Upgraded server from 5.0.19 to 6.0.2.
  3. Upgraded clients to ZCO 6.0.2.
  4. Turned on View -> Toolbars -> Zimbra Connector


Results:

Share detection fails. Clicking on Share Status gives popup "No sharing links have been detected"

Cannot share to or from non-resource accounts. I can share to resource accounts.

Mailbox selection error

This user does not have a Zimbra mailbox. You cannot open the mailbox of a user that does not have a Zimbra mailbox. Please check if this user belongs to a foreign GAL.
Originally attempted with the default GAL strategy as migrated from 5.0.x, i.e., no GAL sync account. Adding a GAL sync account did not improve things.