Android Studio - Lollipop, ADB, & Genymotion issues: Devices won't show up. "adb server is out of date. killing..." [duplicate]

Try running this:

killall -9 adb

That should remove all running traces of the daemon

Edit: Best solution below

This issue may be adb incompatibility with the newest version of the platform SDK. The best solution for this issue with Genymotion is to set the Android SDK within Genymotion to your location. This can be found within the Settings page.

In my case the problem was that I have installed adb tools and fastboot using this command

sudo apt-get install android-tools-adb android-tools-fastboot

From repository, so in this case this executables goes to the /usr/bin/ directory in order that you can use it system-wide ($PATH variable).

In addition, I am running android studio with genymotion configured like in the answer above.

So when I was trying to acces adb (adb shell in my case) it was trying to start another copy of adb process (server) and bind to default port, so the problem was here.

I have just uninstalled the android-tools-adb android-tools-fastboot and added the path to my Android SDK to the $PATH variable