I have about 60 .pfx files for all major assemblies for a project. They are unique files, but I use the same password for all.
After an upgrade from Windows Vista to Win
Solved it!
After the upgrade from Vista to Windows 7 the permissions of the folder containing certificates C:\Users\All Users\Microsoft\Crypto\RSA\MachineKeys had changed.
After changing the permissions granting full access the problem went away.
Let me know if this was helpful to anyone! :)