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)...
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