Hi,@OSR OSR
Thanks for your post.
Is there the 3rd part software installed recently? Since many files are affected, I suspect if there's some lock programs do the operation. Please try to booting into safe mode and check again.
If after that, it is still no clue. I'm afraid you could only use Takeown to assigne ownership to the Administrators group and do the operation.
In addition, if you would like to know how it happened, please also enable auditing. So that you may know who change the folder next time.
Best Regards,
Vicky