Stuck on HTC boot

What type of ROM are you trying to install? If it's a custom one you need a custom recovery (which will require unlocking your bootloader again). If you want a stock ROM (from an HTC ROM Upgrade Utility, RUU) then you can do it in 2 ways:

1) using the RUU as a Windows .exe, put the phone in fastboot mode, connect to a PC, run the RUU on the PC

2) using a ROM .zip, put the file on your sd card, rename it to some special name (which is different for different HTC models, I don't recall what it is for the M8 but a quick web search should find it) and reboot the phone: the bootloader will identify and install it.

Obviously it has to be the right RUU/ROM for your phone, so it's important to know what model of M8 you have (if it's something unusual like a Verizon or Sprint one you do not want to install GSM firmware!). Normally the phone would protect you against installing the wrong software, but as yours has been modified to bypass all such protections (S-Off and that particular CID number) you need to make sure you don't try to install the wrong thing - the flip side of making such mods is that you gain the power to screw things up as well as the advantages of having that flexibility. Of course that assumes you made the modifications, rather than a previous owner.

From your radio version I'd say you have a GSM model (M8_UL), so a European WWE (WorldWide English) ROM/RUU would be OK.

I can't tell you why what you've done hasn't worked because you've not said anything about the file you were trying to install, but most likely is that your .zip file is wrong (e.g. you are trying to install a custom ROM via the bootloader method).

Sorry, busy day, may not be able to get back to you immediately. Bumping this to remind myself (and anyone else who wants).

That first (2 posts above) sounds like a custom ROM, which would be why it didn't work.

So, as I'm briefly online at the moment:

* Custom ROM option: for this you do need a custom recovery. TWRP is generally the recommended one these days. You can find download links and instructions here. You'd want the "no root" method of installation, which requires downloading a TWRP zip from one of the links, installing fastboot on a computer (and drivers if you use Windows), then using fastboot to install the recovery. Since your bootloader is locked you'd need to unlock it again, which will also do a factory reset.

Once you have the recovery installed you just need to put the ROM on your SD card (card reader is simple, or you can use ADB when in the custom recovery), then factory reset from recovery (though unlocking did that) and then select install from the recovery menu.

* Stock ROM option: from the list linked in your last post, you should in principle be able to use any of the M8_UL RUUs. Do not use an M8_WHL one, that would be very dangerous. Unless you have a good reason to do so (like needing a carrier-specific feature - I don't know which carrier you are using it with) I'd choose a generic one like the HTC_Europe one rather than a carrier-specific one (more bloatware!). Because of your CID value you could use any, which is not the normal situation.

I think this should all work with the bootloader locked (as it currently is). In fact the .exe files I think will require it.

I think your phone currently has Lollipop (5.x) firmware, but you can use a Marshmallow (6.x) one if you prefer - it will completely update the phone anyway.

For the .exe files you should download to a PC, connect the phone by USB, put the phone in fastboot mode and then run the RUU. A couple of them are OP6BIMG_xxx.zip files: these you could try putting on your SD card and installing as you attempted before (boot into bootloader and see whether it will just install them). I can't remember whether you need to rename them to just OP6BIMG.zip or whether the name starting like that will be sufficient. Or there is a sequence of fastboot commands you can use to flash an RUU zip file: the instructions in this thread for "method #2: fastboot/RUU mode .zips" should work for flashing an RUU zip (that's why it's called "RUU mode": these are really the instructions for flashing RUUs in zip format).

Sorry, I know that's quite a dense collection of facts, but I'm not sure I'll be online much more tonight and I think everything is in there. But make sure you understand and are comfortable with what you are doing before trying anything, and ask if there is something you don't understand.

Fastboot flashing a RUU.zip would be pretty easy. You have S off and supercid.