[Q] Unable to flash a custom rom (s on, recovery booted via fastboot, htcdev unlock) topic

Hey you pros :)

I am currently a little confused. I just got my HTC ONE m8 (dual boot) a week ago and tried since then to flash a custom rom. I unlocked my phone via htcdev and unlocked is shown. I do not have s off, because I read flashing should also work this way. I tried with twrp 2.8.1.0 on the old firmware (1.45.401.12). I did a full wipe and tried every different way, like booting into recovery via fastboot, flashing the kernel seperately. The newer version of twrp didn't work, and neither did cwm. Then I tried to update the firmware over the normal OTA way and have now version 3.33.401.6. The new twrp is working now, but i again tried all ways known to me and no one worked. Do you guys know what is wrong? I searched over google read different threads, but can't find an answer.

I would really appreciate if you could help me!

Here is the fastboot getvar all:
(bootloader) version: 0.5
(bootloader) version-bootloader: 3.19.0.0000
(bootloader) version-baseband: 1.22.30306251.27G
(bootloader) version-cpld: None
(bootloader) version-microp: None
(bootloader) version-main: 3.33.401.6
(bootloader) version-misc: PVT SHIP S-ON
(bootloader) serialno: ---
(bootloader) imei: ---
(bootloader) imei2: ---
(bootloader) meid: 00000000000000
(bootloader) product: m8_dugl
(bootloader) platform: hTCBmsm8974
(bootloader) modelid: 0P6B64000
(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: 4c3fbd70
(bootloader) hbootpreupdate: 11
(bootloader) gencheckpt: 0
all: Done!

Oh and by the way I tried to flash the Official Nightlys of CM 11 and "[ROM][5.0.2][OFFICIAL] LiquidSmooth v4.0 | Lollipop | [12-25-14]".

Thanks for your help!
xhdev