If you have adb installed as part of your Linux distribution, then your PATH probably points there. Most likely, this is adb - version 1.0.31 (or earlier).
Genymotion comes with its adb in the tools/ directory, which is version 1.0.32 if you are using the latest version of Genymotion, currently 2.3.1. If you have not changed it in the settings, Genymotion will launch a bridge with this copy.
Since adb works in a client-server configuration, the client version must match the daemon version. In your case, they do not, and why you get the adb server is out of date. killing... message adb server is out of date. killing... adb server is out of date. killing... If the daemon is killed or dies, the Genymotion launcher automatically and immediately restarts adb (using its own copy) and therefore your client cannot start a new daemon associated with port 5037.
The solution is to make sure that the version of adb that you invoke on the command line matches the version that Genymotion brings up as a daemon when starting virtual machines. The easiest way to provide this is to simply use the adb binary in the Genymotion tools/ directory.
Note that if you changed the default Genymotion value using your own tools to use custom SDK tools, you will need to use the adb client that matches the version contained in the SDK that you are pointing to. As far as I can tell, KitKat (4.4.4) and earlier SDKs contain adb version 1.0.31, while Lollipop SDKs (5.0) have 1.0.32.
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