Hello Poomba_1,
I don't believe I understood your reply, as you also did not specifiy in which part of my conclusion you disagree.
Let me again try to clarify :
In 16299.xxxx (Windows 10 1709) every next boot after having done an AMD driver update or having run "CCleaner" was fine.
Starting with 17134.1 (Windows 10 1803),
every next boot after having done an AMD driver update or having run "CCleaner", and now after
attempting to install KB4103721, was resulting in a "black screen" (before logon) and the only way to solve/bypass was to boot to "safe mode" (necessary once after having done the AMD driver update or having run
"CCleaner", necessary twice after KB4103721, which furtheron was leading to a rollback of KB4103721).
For MY particular situation I was able to find out, that disabling the "Syncovery VSS helper service" allowed any further boot (after above mentioned "maintenance tasks") to complete without any issue (as in 16299.xxxx).
So again, KB4103721 (the update from 17134.x to 17134.48) is NOT causing the problem, but at the end of the day suffering from an issue already introduced with 17134.1 (Windows 10 1803 final).
Of course I did never state, that every Windows 10 user will run in that problem, as obviously not every Windows 10 user is using "IoBit Advanced System Care" or "Syncovery" (known at the moment to cause the "black screen" situation) ... it is now up to Microsoft
to find out, why those (or maybe other) products are causing (kind-of) compatibility issues in the Windows 1803 boot process.