There's a documented issue when recovering exchange items with VEX when the Exchange CAS is an array behind a load balancer. https://www.veeam.com/kb1936
Although the workaround is simple enough (point to the FQDN of an individual server) I've found a solution that works, specifically for a Kemp loadbalancer and Exchange 2016. It may work for other versions of Exchange and Load Balancers. You do need to be running at least V7.1 of the Kemp firmware.
On the Kemp - Go to System Configuration
-> Miscellaneous Options
-> L7 Configuration
. From here change the 100-Continue Handling
setting from the default RFC-2616 Compliant
to RFC-7231 Compliant