I solved the problem, at least temporarily, as follows:
Navigate to the \Temp directory specified in the error message (C:\Users\username\AppData\Local\Temp\).
Look inside that directory for a subdirectory called "BaseCamp."
Double-click "BaseCamp." You should get an error message saying you don't have access to it, and if so you'll be invited to click a button to grant yourself full access.
Once you click that button, you should be able to start BaseCamp again. It worked for me.
Navigate to the \Temp directory specified in the error message (C:\Users\username\AppData\Local\Temp\).
Look inside that directory for a subdirectory called "BaseCamp."
Double-click "BaseCamp." You should get an error message saying you don't have access to it, and if so you'll be invited to click a button to grant yourself full access.
Once you click that button, you should be able to start BaseCamp again. It worked for me.