[One (M8) Q&A, Help & Troubleshooting] BloodySword:
IT WORKED!!

ckpv5's method was the goal!

You're right, it somehow locked itself up before fastboot could pass any data!

The log:

C:\adb>fastboot flash zip fw_6.12.401.4.zip
target reported max download size of 1830612992 bytes
sending 'zip' (53847 KB)...
OKAY [ 2.382s]
writing 'zip'...
(bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) checking main version...
(bootloader) checking hboot version...
(bootloader) start image[hboot] unzipping for pre-update check...
(bootloader) total_image_number=8

(bootloader) start image[hboot] flushing...
(bootloader) signature checking...
(bootloader) [RUU]WP,hboot,0
(bootloader) [RUU]WP,hboot,99
(bootloader) [RUU]WP,hboot,100
(bootloader) ...... Successful
(bootloader) current_image_number=0

(bootloader) current_image_number=1

(bootloader) current_image_number=2

(bootloader) current_image_number=3

(bootloader) current_image_number=4

(bootloader) current_image_number=5

(bootloader) current_image_number=6

(bootloader) current_image_number=7

FAILED (remote: 90 hboot pre-update! please flush image again immediately)
finished. total time: 6.675s

C:\adb>fastboot flash zip fw_6.12.401.4.zip
target reported max download size of 1826418688 bytes
sending 'zip' (53847 KB)...
OKAY [ 2.615s]
writing 'zip'...
(bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) zip header checking...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) checking main version...
(bootloader) checking hboot version...
(bootloader) total_image_number=13

(bootloader) start image[adsp] unzipping & flushing...
(bootloader) [RUU]UZ,adsp,0
(bootloader) [RUU]UZ,adsp,10
(bootloader) [RUU]UZ,adsp,21
(bootloader) [RUU]UZ,adsp,33
(bootloader) [RUU]UZ,adsp,43
(bootloader) [RUU]UZ,adsp,54
(bootloader) [RUU]UZ,adsp,65
(bootloader) [RUU]UZ,adsp,76
(bootloader) [RUU]UZ,adsp,88
(bootloader) [RUU]UZ,adsp,98
(bootloader) [RUU]UZ,adsp,100
(bootloader) [RUU]WP,adsp,0
(bootloader) [RUU]WP,adsp,100
(bootloader) ...... Successful
(bootloader) current_image_number=0

(bootloader) start image[boot] unzipping & flushing...
(bootloader) [RUU]UZ,boot,0
(bootloader) [RUU]UZ,boot,5
(bootloader) [RUU]UZ,boot,10
(bootloader) [RUU]UZ,boot,15
(bootloader) [RUU]UZ,boot,20
(bootloader) [RUU]UZ,boot,25
(bootloader) [RUU]UZ,boot,30
(bootloader) [RUU]UZ,boot,35
(bootloader) [RUU]UZ,boot,40
(bootloader) [RUU]UZ,boot,50
(bootloader) [RUU]UZ,boot,55
(bootloader) [RUU]UZ,boot,65
(bootloader) [RUU]UZ,boot,70
(bootloader) [RUU]UZ,boot,80
(bootloader) [RUU]UZ,boot,86
(bootloader) [RUU]UZ,boot,95
(bootloader) [RUU]UZ,boot,100
(bootloader) [RUU]WP,boot,0
(bootloader) [RUU]WP,boot,99
(bootloader) [RUU]WP,boot,100
(bootloader) ...... Successful
(bootloader) current_image_number=1

(bootloader) start image[recovery] unzipping & flushing...
(bootloader) [RUU]UZ,recovery,0
(bootloader) [RUU]UZ,recovery,5
(bootloader) [RUU]UZ,recovery,11
(bootloader) [RUU]UZ,recovery,17
(bootloader) [RUU]UZ,recovery,22
(bootloader) [RUU]UZ,recovery,28
(bootloader) [RUU]UZ,recovery,33
(bootloader) [RUU]UZ,recovery,39
(bootloader) [RUU]UZ,recovery,44
(bootloader) [RUU]UZ,recovery,56
(bootloader) [RUU]UZ,recovery,61
(bootloader) [RUU]UZ,recovery,66
(bootloader) [RUU]UZ,recovery,72
(bootloader) [RUU]UZ,recovery,79
(bootloader) [RUU]UZ,recovery,89
(bootloader) [RUU]UZ,recovery,95
(bootloader) [RUU]UZ,recovery,100
(bootloader) [RUU]WP,recovery,0
(bootloader) [RUU]WP,recovery,89
(bootloader) [RUU]WP,recovery,99
(bootloader) [RUU]WP,recovery,100
(bootloader) ...... Successful
(bootloader) current_image_number=2

(bootloader) start image[rpm] unzipping & flushing...
(bootloader) [RUU]UZ,rpm,0
(bootloader) [RUU]UZ,rpm,100
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=3

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) current_image_number=4

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=5

(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) [RUU]UZ,sbl1,0
(bootloader) [RUU]UZ,sbl1,100
(bootloader) signature checking...
(bootloader) verified fail
(bootloader) ..... Bypassed
(bootloader) current_image_number=6

(bootloader) start image[sensor_hub] unzipping & flushing...
(bootloader) [RUU]UZ,sensor_hub,0
(bootloader) [RUU]UZ,sensor_hub,100
(bootloader) [RUU]WP,sensor_hub,0
(bootloader) [RUU]WP,sensor_hub,4
(bootloader) [RUU]WP,sensor_hub,8
(bootloader) [RUU]WP,sensor_hub,12
(bootloader) [RUU]WP,sensor_hub,16
(bootloader) [RUU]WP,sensor_hub,20
(bootloader) [RUU]WP,sensor_hub,24
(bootloader) [RUU]WP,sensor_hub,28
(bootloader) [RUU]WP,sensor_hub,32
(bootloader) [RUU]WP,sensor_hub,36
(bootloader) [RUU]WP,sensor_hub,40
(bootloader) [RUU]WP,sensor_hub,44
(bootloader) [RUU]WP,sensor_hub,48
(bootloader) [RUU]WP,sensor_hub,52
(bootloader) [RUU]WP,sensor_hub,56
(bootloader) [RUU]WP,sensor_hub,60
(bootloader) [RUU]WP,sensor_hub,64
(bootloader) [RUU]WP,sensor_hub,68
(bootloader) [RUU]WP,sensor_hub,72
(bootloader) [RUU]WP,sensor_hub,76
(bootloader) [RUU]WP,sensor_hub,80
(bootloader) [RUU]WP,sensor_hub,84
(bootloader) [RUU]WP,sensor_hub,88
(bootloader) [RUU]WP,sensor_hub,92
(bootloader) [RUU]WP,sensor_hub,96
(bootloader) [RUU]WP,sensor_hub,100
(bootloader) ...... Successful
(bootloader) current_image_number=7

(bootloader) start image[sp1] unzipping & flushing...
(bootloader) ...... Successful
(bootloader) current_image_number=8

(bootloader) start image[tz] unzipping & flushing...
(bootloader) signature checking...
(bootloader) ...... Successful
(bootloader) current_image_number=9

(bootloader) start image[wcnss] unzipping & flushing...
(bootloader) ...... Successful
(bootloader) current_image_number=10

(bootloader) start image[radio] unzipping & flushing...
(bootloader) trying to rename MBA
(bootloader) ...... Successful
(bootloader) current_image_number=11

(bootloader) start image[rcdata] unzipping & flushing...
(bootloader) ...... Successful
(bootloader) current_image_number=12

OKAY [ 64.004s]
finished. total time: 66.622s

I want to thank you all guys. I mashed the button on EVERY post.

Kindest regards from Germany!

But the journey doesn't stop there!

When I want to S-OFF, should I first unlock the bootloader or should I just SunShine?

Edit: WTF...

It now boots into ... CyanogenMOD.. oO

Edit2:

The volume rocker often bounces, so I selected the wrong menu entry. It is now showing the CyanogenMOD robo animation but nothing happens.