For many, the August 5th release of Windows 10's first bug fix update, the installation was smooth sailing. However, there's a small contingent of users who haven't had much luck getting the cumulative update (CU) installed. But, that number is growing, according to a thread in the Microsoft Answers forum.
The resulting problem is that the CU fails to install, gets to a different percentage of completeness each time, but then rolls back – continuously. The error message Package KB3081424 failed to be changed to the Installed state. Status: 0x80070020 can be found in the Windows 10 Event Viewer, but like most Microsoft error messages, doesn't give much additional detail. Searching the web shows that this error message is attributed to a litany of problems, none of which give any resolutions to this particular situation.
The Answers forum thread does give some relief, but comes in the way of a registry modification where old profiles, remnants of an upgraded machine, could be the cause. Per forum participant MichaelJTaylor, the information is contained in the HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\ProfileList key. This key contains the Security Identifiers (SID) for old accounts. Removing the SID's that no longer exist on the machine allows the CU to install.
BIG Warning: Only try this if you're completely familiar with modifying the Windows registry. I'm sure Microsoft will come up with a solution soon. If you delete the wrong SID, or even accidentally wipe out the entire ProfileList key, you won't be able to log back into Windows and will require a full reinstallation of Windows 10.
Incidentally, the tool Microsoft released to block specific updates doesn't work for this cumulative update.