UserInstall directory wrong for ordinary users
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Windows 7 64 bit, MikTeX 2.9.5857 installed from administrator account for "all users". On the fly (and I suspect other) package installation does not work. The per user install directory is under the administrators profile, e.g. "C:\Users_AdminUser\AppData\Local\Programs\MiKTeX 2.9", for all users. And other users will of course not have write access.
Setting HKEY_CURRENT_USER\Software\MiKTeX.org\MiKTeX\2.9\Core to "C:\USers_UserAccount\AppData\Local\Programs\MiKTeX 2.9" seems to fix things but MikTeX should really pick up %USERPROFILE% or something like that.
This bug is fixed for me with the 2.9.5872 basic installer.
(Unfortunately now MiKTeX is by default only installed for the current user, no matter if the user is the admin, see bug https://sourceforge.net/p/miktex/bugs/2436).