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 “adb server is out of date.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.