Funny thing ... it seemingly worked for me now.
The problem is that I cannot tell you how.
Here's what I did - maybe it can help someone to think of things to try:
As I said, I removed my AVAST Free. Instead I installed Microsoft Security Essentials, which didn't make much of a difference regarding this issue. So I tried to install KB4474419 a few more times and then thought like "screw that, W7 won't be supported
for long anyway, let's get Windows 10".
But as I tried earlier (version 1703 or sth like that) and encountered several problems (older hardware), I first booted up Acronis True Image 2016 and got myself a clone of my entire system disk.
So I installed Win 10, had the same problems as last time and decided to go back to 7.
During the installation of W10 I also came to remember I got this old small HDD plugged in on which I once had Ubuntu running. And yeah, right, the GRUB loader always appeared for like a second when booting. Now it didn't, as the Win 10 installation routine
must have overwritten it in the boot sector or whatever.
So I disconnected the Linux HDD, booted Acronis again, recovered my system disk using the clone I created before and - more or less just for fun - started the installation of KB4474419 from the .msu on my desktop again.
Well ... it just worked this time.
The system asked for a reboot, I confirmed, it rebooted and no "error" and stuff ... it seems to be installed.
Sorry, German. Just for you to see I don't make up stories or such. ;)
