That unfortunately does not help to understand the issue.
As you can see there is the snapshot unmount command
[23:05:01.921] Dismounting LUN [C:\Program Files\NetApp\SnapManager for Exchange\SnapMgrMountPoint\MPDisk420] of Snapshot [exchsnap__srvex01_12-19-2017_23.00.22__daily]...
[23:05:24.781] Dismount snapshot successfully completed.
and then another mount for the next verificaiton
23:05:36.375] Mounting Snapshot [exchsnap__srvex01_12-19-2017_23.00.22__daily] for LUN H created from Computer SRVEX01
[23:05:36.375] Mount point directory [C:\Program Files\NetApp\SnapManager for Exchange\SnapMgrMountPoint]
something happen in between..
which SD version are you using? is this in IMT with the other software? https://mysupport.netapp.com/matrix/#search
do you have MPIO enabled?
what do you see in the event viewer and in the sanpdrive logs at the same time of the failure?