I had the same problem trying to reinstall Steel Storm: Burning Retribution.
Error 401 is an authentication error.
I just found out to get it back for my case I hope it will help.
First, you need to know that your authentication details for the PPA are stored in
Second you need to verify that the details are correct by going to https://launchpad.net and connecting with the details you used to purchase the software.
Once logged in click on your name if the right top corner (next to the logout button).
In the "Personal package archives" section click on "View your private PPA subscriptions".
On the right side of the line corresponding to your purchase click on "view"
You should now see something that looks like this:
deb https://USERNAME:PASSWORD@private-ppa.launchpad.net/commercial-ppa-uploaders/PURCHASE/ubuntu YOUR_UBUNTU_VERSION_HERE main
PASSWORD respectively match login and password in your
/etc/apt/auth.conf is updated with the correct details, run as
sudo apt-get update from the terminal and check if you still get error messages, if you don't you should now be able to re-install the game.
Another problem that you can have is if your game doesn't have a PRECISE version yet, in that case you need to go in
/etc/apt/sources.list.d/ and modify the file containing the description of the private PPA (in my case
Example in my case:
I had to replace this line:
deb https://private-ppa.launchpad.net/commercial-ppa-uploaders/steel-storm2/ubuntu precise main
deb https://private-ppa.launchpad.net/commercial-ppa-uploaders/steel-storm2/ubuntu oneiric main