Question:
Below about says it all. I tried SysRes from two points, both with same failure.
System Restore did not complete successfully. Your computer's system files and
settings were not changed.
Details:
System Restore failed while restoring the directory from the restore point.
Source: AppxStaging
Destination: %ProgramFiles%\WindowsApps
An unspecified error occurred during System Restore. (0x80070091)
The two attempts were from: choose a different restore point>show more restore points type Backup. I really need this function to work. Any solutions out there?
Answer:
Renaming folder WindowsApps, reboot the system and try a system restore should cure the issue.
1.下載一個win10的iso鏡像文件（官方原版的）
2.用ultraiso將鏡像寫入U盤(方法自己百度)
3.利用BIOS提供的啟動順序切換，從U盤啟動，耐心等待
4.選擇完語言下一步，第二個畫面左下角，點“修復計算機(R)"，點“疑難解答”，點“命令行提示符”
依次敲入命令：
c:
cd "Program Files"
md "WindowsApps-"
xcopy "WindowsApps" "WindowsApps-" /s /e /y /h
rd "WindowsApps"
關閉命令行窗口，點“疑難解答”，點“系統還原”，再執行就OK了。
I've described a solution here Windows 10 Version 1607: System restore error 0x80070091 [Fix].
Addendum: It seems that the issue has been fixed in Windows 10 Creators Update. And there is a batch file to fix the issue. Further details may be found here:
Windows 10: News about System restore error 0x80070091