I had this issue with several systems in my environment with 1709 and we discovered its due to boot partition corruption. The fix was to decrypt the disc offline using DETech and then using a Windows disk or MS DaRT disc run command prompt following the below instructions.
Run DISKPART DISKPART
List disks if there are multiple disks LIST DISK
Select the primary disk SEL DISK #
List the volumes and partitions to see the EFI boot partition LIST VOL
Select the EFI partition which should be Fat32 SEL VOL #
Assign it a letter ASSIGN LETTER=F:
Exit the DISKPART EXIT
Change disk to the assign partition F: or cd F:
Format the EFI Partition format F: /FS:FAT32
Fix the boot settings bcdboot C:\Windows /s F: /f UEFI
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