Well, I've found the cause of it... but no idea how to fix it "properly".   Exporting the XML config I found a list of all the registered mobile devices, attached to a user account and with a: <EAS-backend> field set (to the incorrect URL)   So deleting registered devices under System>Status>Devices fixes the issue.   No clue where it inherits that from... could be something from their MDM solution (Airwatch), could be from AD/Mailbox, could be an old legacy field from older versions of the config (upgraded all the way from version 5 or 6 IIRC)...       ... View more