adb server version doesn't match this client

Are you using Genymotion for a virtual device?

if yes this error probably came out because ADB from Genymotion conflicted with your ADB from Android SDK(using same port number), to fix this simply go to settings => choose ADB tab => click on the option Use custom Android SDK Tools and set your SDK folder

after you configure this, try to restart your adb by going into folder platform-tools which adb placed and do this command:

• ./adb kill-server
• ./adb start-server

*tips: You may close the process of Genymotion before running the command above

Hope this help.

I uninstalled Dell PC Suite and HTC Sync from my computer and this problem went away.

EDIT: To elaborate a bit on the cause of this problem: HTC sync comes with an ADB server of its own. And it updates your PATH environment variable to point to its version of the server. Edit the PATH variable and remove the reference to the HTC Sync directories. Now you're using Google's ADB again.

I find I occasionally need to explicitly kill and restart the server manually:

adb kill-server
adb start-server

This shock is generally enough to get it back on its feet again, and prevents repeated adb server is out of date. killing... messages.