I have found and fixed this error which was actually due to a corruption with the Task Scheduler on Windows 10 (which took several attempts at upgrading from Windows 7).
1) After noticing that the log file ending with the line "Creating scheduled task if doesn't already exist" I tried to change the setting for "Automatic Catalog Update" on the "MySQL Installer Options" screen, but whenever I pressed the CLOSE button that would hang as well, and I had to terminate the task using Task Manager.
2) I have a 2nd PC which was working perfectly, so I exported the task from that Scheduler and imported it into the faulty PC, but when I tried to update the user account from PC#2 to PC#1 it failed to do anything when I pressed the OK button.
3) I suspected a fault with the Task Scheduler, so I downloaded and ran the RepairTasks program from
https://repairtasks.codeplex.com/releases/view/617575 which found and fixed one problem. When I reran step #2 above the OK button did not hang, but it did produce an error message saying that a task with the name "ManifestUpdate" already existed even though it did not show up in the list of tasks.
4) I then compared the registry entries for "ManifestUpdate" on both PCs, and I noticed that the entry in \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks\ on the faulty PC had fewer lines, so I deleted this entry as well as the one in \HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\S-1-5-18\Components\
5) I then reran step #1 above, and this time it worked. I then reran the latest msi installer and that worked as well!