A disk read error occurred BOOTMGR is missing . BOOTMGR is compressed. Press Ctrl+alt

[NiKiZe]

You should have more information then just the BOOTMGR message.

If you see that message in wireshark then that is just the contents of the packets and is no issue on it's own (it's data on disk)

Also the iSCSI server itself will never know anything about BOOTMGR.

Are you running a windows installation on a iSCSI disk via wimboot which fails on reboot?

Might help us understand your issue if you could post an image of this message.