In the latest adb update the adb server is out of date. killing... message has been replaced with more informative adb server version (%d) doesn't match this client (%d)
So this solution is applicable to both.
The root cause of the error is that your system has multiple adb binaries of different versions) installed. These adb binaries may come from different sources - distributed together with some development tools or even installed from your linux distribution official repository. I would strongly advise against using any of those sources. The official source of the most recent version of adb is the platform-tools package from the Android SDK. The adb tag info page contains the direct links to the package.
So if you have Android SDK already installed - just update the platform-tools package to the latest version using SDK Manager. If not - download and unpack the contents of platform-tools.zip file for your platform to a folder. Add that folder to your $PATH. Now kill all running adb processes and make sure to find all other copies of adb and delete them. In rare cases some software might stop working. In that case you would need either change that application's internal setting telling it where to find the new binary or if the application uses hard-coded adb location - just create a symlink to the new location.
That's it. No more server/client version mismatches. Although you might need to repeat the procedure after installing another software which comes prepackaged with its own adb copy.
