Started to receive the error: Error signing assembly -- Unknown error (8013141c)

This is a permission issue. To resolve this error; open the following folder - “C:\Documents and Settings\All Users\Application Data\Microsoft\Crypto\RSA\MachineKeys”, and give your user (whatever account you use to log onto your computer and work on Visual Studio) full control. You can do it by right-clicking on the MachineKeys folder and select properties, and in the security tab, make sure the current user have full control.

Building on Mangist above, his solution worked for me, but I have Windows 10 so MachineKeys are in a different folder. I gave myself full control (probably only need read/write) of the following folder:

C:\Users\All Users\Microsoft\Crypto\RSA\MachineKeys

It said access denied for some of the keys, but once the process was done, I no longer see the Error signing Assembly issue.

Run Visual Studio with elevated permissions, as an Administrator.