Slitherine

I just loaded Panzer Corps yesterday. I can play single player fine. I tried to play multiplayer and it tells me that I need the latest update. It loads a file for 10 to 12 minutes and gets to 1 second left. Then it displays a message that there was a download error. Any idea?

Welcome.
Try to update it manually.
Here's a list with mirrors:
http://www.matrixgames.com/products/399/downloads/
After having download the update file, just execute it and the update will be installed.