Various errors: Background image / Windows Defender / PIN login

Hey folks,

I have just set up my machine with a clean install of Windows 10.

After having spent the last 2 days or so installing all the programs and tools, and applying smaller registry hacks (mostly to the file explorer), I have - out of the blue - run into a couple of odd errors.

I came to notice them when I was cleaning up my wallpapers folder and tried to set 2 images as background pictures (dual monitor setup).

1.) Windows background changed to solid black.
Upon checking the settings, it turned out that I was unable to change it back to "Image". Whenever I picked an image and hit "apply", it would fall back to "solid color".

Hence, I decided to reboot, when:

2.) I couldn't log in to my account using the PIN method.
It worked with entering the password - but the PIN method wasn't even available in the login screen.
Looking at the settings, it seems like it has been disabled (see Screenshot), but whenever I hit the "Hinzufügen" button, it would do nothing. (update: I tried again and had to re-enter my Windows password; afterwards, again, nothing..)

3.) On checking the systemtray, I noticed that Windows Defender was active again.
Before, I have installed Avira Antivirus and Defender has deactivated itself without a problem. Also, in the Security settings, now, it reads that the system is protected by Windows Defender again (was Avira before).

I don't know if there are possibly more error, that I didn't spot yet. I ran the repair service (don't know it's english name, sry), but besides some strange errors with the Update service, it didn't fix anything; also, the errors still exist.
I was able to fix the 1.) Wallpaper issue, somehow..

But the rest remains a mystery to me and I have no idea what else to do. A system restore is the last resort, since it would void a full days work of installing programs and customizing the system.

So, if anybody has any ideas and is willing to help me out, I would really appreciate it

Regards,

Jan

edit:
Windows is running 1607 with all the available Updates installed.
Also, my only suspicion would be that the security tools might have interefered; but I have never heard that they would have such an effect. I am using Avira Antivirus (Pro), Malware Anti-Bytes (Pro) - and apparently also Windows Defender. I have disabled all of them, but the outcome won't change..

Another update:
3D Builder suddenly appeared back as well. First, I noticed it in the context menu, now it has even nested itself into the start menu again.

Also, the icons of the Windows apps have changed and appear, broken..