File has unexpected size (32768 != 21850). Mirror sync in progress?

Symptoms

Running 'apt update' results in errors such as:

CONFIG_TEXT: File has unexpected size (32768 != 21850). Mirror sync in progress?

Cause

Invalid local cached data within the apt package manager.

Resolution

1. Ensure that apt has the latest data available:

   # apt update

2. Attempt the upgrade again:

   # apt upgrade