[One (M8) Q&A, Help & Troubleshooting] swehack:
Ok so now I start with an HTC that is RELOCKED, with TWRP installed. I use TWRP to wipe dalvik, cache, data just in case. Software status is modified.

This is the output of fastboot oem lock.

(bootloader) [PGFS] partition_update_pgfs: pg1fs_security
(bootloader) TZ_HTC_SVC_ENC ret = 0
(bootloader) [PGFS] partition_update_pgfs: pg2fs_sec_recovery
(bootloader) Lock successfully...
(bootloader) deinit_lcd
(bootloader) mipi display off
(bootloader) mdp_clock_off
(bootloader) turn off fd8c2308 failed 1 times. Try again...
(bootloader) disable_mmss_clk done
(bootloader) pll1_disable done
(bootloader) TZ_HTC_SVC_DISABLE ret = -1610744452 (0x9FFDFD7C)
(bootloader) [CRITICAL] SPMI write command failure: cmd_id = 0, error
(bootloader) = 4
(bootloader) [CRITICAL] SPMI write command failure: cmd_id = 0, error
(bootloader) = 4
(bootloader) [ERR] Cannot halt SPMI arbiter!!
(bootloader) [INFO] Rebooting device

Disable hiding known file extensions in Explorer.exe.

Verify checksum of RUU.zip (469c3b8504ce2a2c8aae0fde17bfe028) and htc_fastboot.exe (7343c5bcec64373f83d69bd074954759) with fciv -md5.

Boot into fastboot.

Check fastboot devices.

Run fastboot oem rebootRUU so it reboots into the silver HTC logo screen

Run htc_fastboot.exe flash zip RUU.zip

Output from htc_fastboot.exe is at first.

sending 'zip'... (53965 KB) OKAY
sending time = 2.627 secs
writing 'zip'... (bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) start image[hboot] flushing...
(bootloader) [RUU]WP,hboot,99
(bootloader) ...... Successful
(bootloader) current_image_number=2

(bootloader) current_image_number=5

(bootloader) current_image_number=7

FAILED (status malformed (0 bytes))
Execution time is 14(s)

I try again and get a longer run.

sending 'zip'... (53965 KB) OKAY
sending time = 2.625 secs
writing 'zip'... (bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) [RUU]UZ,rpm,0
(bootloader) signature checking...
(bootloader) current_image_number=2

(bootloader) [RUU]UZ,adsp,10
(bootloader) [RUU]UZ,adsp,33
(bootloader) [RUU]UZ,adsp,54
(bootloader) [RUU]UZ,adsp,76
(bootloader) [RUU]UZ,adsp,98
(bootloader) [RUU]WP,adsp,0
(bootloader) ...... Successful
(bootloader) start image[pg2fs_spcustom] unzipping & flushing...
(bootloader) [RUU]UZ,pg2fs_spcustom,38
(bootloader) [RUU]UZ,pg2fs_spcustom,100
(bootloader) current_image_number=4

(bootloader) [RUU]UZ,wcnss,22
(bootloader) [RUU]UZ,wcnss,68
(bootloader) [RUU]UZ,wcnss,100
(bootloader) [RUU]WP,wcnss,100
(bootloader) current_image_number=5

(bootloader) [RUU]UZ,tp,100
(bootloader) current_image_number=6

(bootloader) [RUU]UZ,tp,100
(bootloader) current_image_number=7

(bootloader) [RUU]UZ,sensor_hub,100
(bootloader) [RUU]WP,sensor_hub,12
(bootloader) [RUU]WP,sensor_hub,68
(bootloader) [RUU]WP,sensor_hub,92
(bootloader) verified fail
(bootloader) current_image_number=10

(bootloader) ...... Successful
(bootloader) start image[sbl1] unzipping & flushing...
(bootloader) verified fail
(bootloader) current_image_number=12

(bootloader) current_image_number=13

(bootloader) current_image_number=14

(bootloader) ...... Successful
(bootloader) start image[rcdata] unzipping & flushing...
(bootloader) current_image_number=16

OKAY
sending 'zip'... (549729 KB) OKAY
sending time = 17.657 secs
writing 'zip'... (bootloader) adopting the signature contained in this image...
(bootloader) signature checking...
(bootloader) rom parsing start ...
(bootloader) rom parsing finish ...
(bootloader) zip info parsing...
(bootloader) checking model ID...
(bootloader) checking custom ID...
(bootloader) checking main version...
(bootloader) checking hboot version...
(bootloader) total_image_number=1

(bootloader) start image[system] unzipping & flushing...
(bootloader) [RUU]UZ,system,0
(bootloader) [RUU]UZ,system,8
(bootloader) [RUU]UZ,system,16
(bootloader) [RUU]UZ,system,24
(bootloader) [RUU]UZ,system,32
(bootloader) [RUU]UZ,system,40
(bootloader) [RUU]UZ,system,48
(bootloader) [RUU]UZ,system,56
(bootloader) [RUU]UZ,system,64
(bootloader) [RUU]UZ,system,72
(bootloader) [RUU]UZ,system,80
(bootloader) [RUU]UZ,system,88
(bootloader) [RUU]UZ,system,96
(bootloader) [RUU]UZ,system,100
(bootloader) [RUU]WP,system,0
(bootloader) [RUU]WP,system,8
(bootloader) [RUU]WP,system,16
(bootloader) [RUU]WP,system,24
(bootloader) [RUU]WP,system,32
(bootloader) [RUU]WP,system,40
(bootloader) [RUU]WP,system,48
(bootloader) [RUU]WP,system,56
(bootloader) [RUU]WP,system,64
(bootloader) [RUU]WP,system,72
(bootloader) [RUU]WP,system,80
(bootloader) [RUU]WP,system,88
(bootloader) [RUU]WP,system,96
(bootloader) [RUU]WP,system,100
(bootloader) ...... Successful
(bootloader) current_image_number=0

After that it just hangs and nothing happens, on the HTC screen I see the silver logo with a full green progress bar under it.

The first time I came here I tried to issue fastboot reboot in another terminal but that gave me the following text from htc_fastboot.

FAILED (status malformed (0 bytes))
Execution time is 261(s)

So this time I'm just waiting to see if anything else happens, only a few minutes so far.