I can only quote from the other thread I referenced that repair of the boot configuration enabled Windows 10 to install ......
For what it's worth - in case someone finds this thread: I finally managed to upgrade my box to Win10! As turned out the \Boot\BCD file on the boot manager partition of my system must have been corrupted, even though my system had always booted OK and even though issuing a "bcdedit" command on the recovery console had only shown valid entries.
I only realized that nevertheless something must be fishy with that file, when I tried to do a "bcdedit /export <file>" and always got an error message "invalid handle". A tool to visually edit BCD files that I then tried to inspect the file always crashed when opening that file - another indication that something was wrong with that file. So I finally renamed the file and recreated a new BCD file (using "bootrec /RebuildBCD"). That file allowed to be exported and inspected.
So, after fixing that I gave the Win10 install another try and - fair enough - it worked immediately.
If only had the upgrade procedure emitted the faintest hint in that direction this would have spared me at least 15hrs of search and experimentation! The guy who dared to write this idiotic error message "something happened" should get fired, if not worse! A big MINUS 10 for this worst Windows update experience ever