[Mate 8 Q&A, Help & Troubleshooting] Patneu:
Everything fixed.

Ok, so I'm a little late on this, but because some recent circumstances forced me to mess with my phones firmware again - despite my former new-found resolutions :angel: - I dug into this mess again and finally found the solution for the last unsolved problem I encountered: the dreaded 'image verification error'. :D So I thought, I'd share it with anyone else out there, who may still be struggling:

If you followed my original instuctions, maybe you got stuck at the same point I did: I originally wanted to install a more recent custom ROM based on Android Nougat, but couldn't for the hell of it flash a working TWRP, because of 'image verification error', when I was on a stock Nougat ROM (B5XX-based) and finally settled for my old B1XX-based Ephemeral Mate 8 ROM once more.

So here's what I did to get a working TWRP flashed with a B5XX-based stock ROM (again, no guarantees, just what worked for me):

1. Depending on how badly you messed up before or at what point you currently are, first you may want to follow some of my original instructions 'til step 5. (All links are still working, as far as I can see.) You then should be good to go with a working B1XX-based stock ROM as a basis for what where going to do now. :fingers-crossed: If you already are, great. :good:

1.1: Attention! If you don't already have a working B1XX-based stock ROM, you may want to slightly differ from my original instructions: instead of buying the 15 credits for DC Phoenix in step 3, you could directly choose the "DC-Phoenix + HCU timed license" option for 19 bucks, so you're already set for everything we're doing later and can mess with your phone as much as you want for the next 72 hours - it may come cheaper in the end and it's easier in any case.

1.2: You may notice afterwards, that if you start adb and type 'adb devices', that the serial number of your device changed to something generic like '0123456789ABCDEF'. We'll fix that later on, along with the 'image verification error', but be sure to note your devices original S/N! You can find it on a sticker at the side of the box your phone came in or in the phones 'Settings' -> 'About phone' -> 'Status' -> 'Serial number'. I don't know where else to recover it, if you lost it. :(

2. Once you have a working B1XX-based stock ROM as a basis, update it to the latest B5XX-based one (was B582 for me) via 'Settings' -> 'System update'.

2.1: Attention again! You may want to check 'Developer options' -> 'Enable OEM unlock' first, since it was greyed out (while still checked) for me once I updated to B5XX. Don't know if it's necessary to have it enabled, but better safe than sorry so you don't have to start over. (If you don't see 'Developer options', tap 'About phone' -> 'Build number' a few times.)

3. Download:

3.1: HCU Client from here. (Thanks Petr S. for putting me on the right track!)

3.2: Unofficial TWRP for EMUI 5.X from here. Found here. (Official TWRP versions may work for you, but didn't for me.)

4. If you didn't already, buy a sufficient number of credits (don't know how much it will be) to use HCU Client from here or directly choose the "DC-Phoenix + HCU timed license" option for 19 bucks to get the all-round carefree package. ;)

5. Unpack and start HCU Client and login with your DC-Unlocker credentials.

6. Connect your phone to your PC and enable 'Developer options' -> 'USB debugging' (don't know if necessary, but I did).

7. As instructed by HCU Client, type '*#*#2846579#*#*' (minus the quotes) into your phones dialer and select '1. Background Settings' -> '2. USB Port Settings' -> 'Manufacture Mode'. If your phone should be rebooting somewhere in the process, you may need to do this again.

8. Now click 'Read phone info' in HCU Client ('Autodetect' should be checked already) and wait for it to complete. (If you didn't buy a sufficient number of credits or the mentioned license before, HCU Client will simply get stuck instead of just telling you - which I found a little odd. :rolleyes:)

9. Click 'Repair UMTS' (acknowledge the warning) and type the original serial number of your device (not the '0123456789ABCDEF' one!) into the 'SN' textfield, check it and hit 'Repair'. (I also retyped the 'IMEI' and 'IMEI 2' numbers, but I guess that was unnecessary.)

10. If now you hit 'Read phone info' again, all the textfields under 'Repair UMTS' (except 'WiFi MAC' and 'Bluetooth MAC') should be filled with the right values. Then click 'Unlock' -> 'Read Bootloader code', note the bootloader code. Your phone will shutdown.

11. Power up your phone again, make sure 'Settings' -> 'Developer options' -> 'USB Debugging' and 'Enable OEM unlock' are still checked and bring it into fastboot mode, then open a command prompt and cd to where your 'Minimal ADB and Fastboot' (or whatever you're using) is installed, if you didn't already, and type 'fastboot devices'. You should see your device with the original serial number again.

12. Unlock your device again with the command 'fastboot oem unlock ' followed by the bootloader code you noted before. Now you should be able to flash the unofficial TWRP you downloaded with 'fastboot flash recovery twrp-3.1.1-1-next.img' (assuming you copied the TWRP image file to your 'Minimal ADB and Fastboot' directory for convenience and didn't rename it).

13. Type 'fastboot reboot' and immediately bring your phone into recovery mode. You should now have a working TWRP with a B5XX-based stock ROM and should be able to flash anything you'd like. :fingers-crossed::good:

You should also be able to flash a Nougat-based custom ROM now, though I didn't try that and settled for the stock ROM rooted with Magisk for personal reasons.

A last word of warning: Be sure to frequently make backups in TWRP before you flash or change anything important like installing or updating Magisk or Xposed modules, because I noticed my system to be a little... wonky at this point.

Don't know if that's related to the way I unbricked my phone with all these DC-unlocker tools or just using Magisk or whatever, but several times after changing something important and rebooting, my phone kind of crashed into fastboot mode with some warning about 'native_boot_fail' and wouldn't boot again until I made a factory reset in recovery and started all over (the first time while writing this guide :silly:). Not so bad, as it doesn't happen randomly as long as I don't make any major changes to the system, but you wouldn't want to retype your Google password and setup your system again everytime like I had to do, would you? ;) So, yeah, just make a backup in TWRP before you change anything root-related - and tread a little lightly, as in: don't make to many changes at once - and you should be fine. :)