It may be possible to start the computer in safe mode.
At any rate, oney cause is that the Registry Value "PublisherPolicyChangeTime" in
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\SideBySide had it's data type changed to a String when it should be a QuadWord.
You may have already found this in your search, and you get the error when trying to run regedit so you cannot change it from that program.
Are you able to run command prompt? (Windows Key+R, "cmd.exe", Enter)
if so, you can confirm my hypothesis by using the following command:
reg query HKLM\Software\Microsoft\Windows\CurrentVersion\SideBySide /v PublisherPolicyChangeTime
it should show something similar to the following:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\SideBySide
PublisherPolicyChangeTime REG_QWORD 0x1d08a80c8540fbd
However if it says REG_SZ we'll have to fix that.