Help needed - M8 keeps rebooting

Hello at All,

I need help with M8 of a friend of mine. He got OTA Update to MM, now it keeps rebooting itself.
I googled and tried to find informations at xda, but I can not find a solution.

I am able to get access into Bootloader, Fastboot and Marshmallow, but after a few seconds it restarts itself.
Just enough time to get to developer options before it chrashes.

Boootloader is locked, S-ON. Factory reset via bootloader does not resolve the problem.

here is fastboot getvar:
(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: 1.29.214500021.12G
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 6.12.401.4
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: x
(bootloader) imei: x
(bootloader) imei2: Not Support
(bootloader) meid: 00000000000000
(bootloader) product: m8_ul
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B10000
(bootloader) cidnum: HTC__102
(bootloader) battery-status: good
(bootloader) battery-voltage: 0mV
(bootloader) partition-layout: Generic
(bootloader) security: on
(bootloader) build-mode: SHIP
(bootloader) boot-mode: FASTBOOT
(bootloader) commitno-bootloader: 76df2b54
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0

thanks for help!

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