ADT: The connection to adb is down, and a severe error has occured

The Android Developer Tools (ADT) version of Eclipse has a so-called “Android Virtual Device Manager”, available through the “Window” menu. It essentially holds a list of virtual device configurations. A highlighted configuration can be started.

If no configuration is started, running an application will result in the following error message:

[2013-05-10 13:54:45 - MyFirstApp] Android Launch!
[2013-05-10 13:54:45 - MyFirstApp] The connection to adb is down, and a severe error has occured.
[2013-05-10 13:54:45 - MyFirstApp] You must restart adb and Eclipse.
[2013-05-10 13:54:45 - MyFirstApp] Please ensure that adb is correctly located at '/opt/adt-bundle-linux-x86_64-20130219/sdk/platform-tools/adb' and can be executed.