Updating to 1809 - Black Screen with Cursor :(

Hey!

I've been trying to Update to my machines to 1809, but with barely any success so far.
The Issue im facing right now, is that all local accounts (new ones and old ones), face a roughly 90 second black screen after logging in. The cursor is visible in the meantime and I can open the taskmanager in the background for example, but all windows will fail to show until the ~90 seconds have passed. After that, everything is back to normal. While this error occurs on local accounts, domain users are not affected at all, which makes me wonder where the Issue persists from - I would have expected Issues the other way around..

Things I've tried so far:
Disable App-Readiness;
Ensure that explorer.exe is beeing called as the in the registry as well as reneweing that entry as suggest in several posts;
All of the DISM, CHKDSK, SFC /scannow and so on commands, no changes here.
Updating from 1709 to 1803 to 1809 instead of 1709 to 1809 (This problem does not exist after updating to 1803);
The usual Driver and Runtime Stuff, updating them, fully removing them, testing them in different constellations.;
Disabling Fast-Boot;
Booting into Secure Mode - Login works fine here;
Performing a "clean mode" as suggested by Microsoft - Error persists;
Disabling any Antivirus - No Changes;

I figured that the Issue does not come from the explorer.exe beeing delayed but the Sihost.exe beeing at fault here - If I execute the sihost.exe (blind since the screen is still black) within the 90 second window that the screen stays black, I can see the screen flicker for half a second, showing the desktop as it should be, before returning into beeing dark. If I execute the Sihost after the 90 seconds, so when things operate normally, the screen will go black again for the 90 seconds it takes. There is not much variation within the black screen times, whole login takes between 92 and 94 seconds (measured several times), starting the sihost after everything works takes exactly 90 seconds - Could it be running in some sort of timeout? Where should I check?

Anyone else facing this or an similar Issue?