Thank you!
So the flash worked now with multiple suggestions combined.
The complete solution is:
[Solution of problem1]
Download the Android SDK from SDK Platform Tools release notes | Android Developers
Replace the original fastboot.exe from the provided Sailfish package with the same-named file from the Android SDK
Open md5.lst in Notepad++ and remove line 4
While in the folder with all the files, write the following to the address bar of the Explorer: fastboot.exe snapshot-update cancel
Then continue from guide step 8.4 again
[Problem 2]
Right now I am trying to boot the phone but it seems to be stuck at “Encrypting user data, please wait”. I hope this will eventually come to an end…
Edit:
[Solution of problem 2]
If the phone during your restart at step 8.7/8.8 is taking too long to encrypt user data, do the following:
Force-stop and restart booting (as in step 8.7)
When asked for a security code after that, use 00000 as your code
I would highly recommend to change the security code under Settings → Device lock
Note that the security code must contain at least 5 digits.
If your phone starts to behave weirdly during initial usage/introduction, repeat force-stop and restart.