error on line 1 at column 1: Document is empty

I'm not sure why this happens but every now and then the invalidator password gets corrupted.  When that happens, the following error will be seen when you try to access portal:

This page contains the following errors:
error on line 1 at column 1: Document is empty
Below is a rendering of the page up to the first error.
FIX:
Re-enter the invalidator password for Webcache and Portal.
Portal WebCache