I'm currently experiencing a few issues using ADB with Debian Stretch. Whenever I issue an ADB command, I get the following message:
$ adb root
adb server version (39) doesn't match this client (36); killing...
* daemon started successfully *
Normally this doesn't cause any issues other than a 2 second delay while the ADB server restarts, but when I'm using adb shell the connection will occasionally drop, which is annoying. Note that this is different to other questions such as this one as the server version is greater than the client version.
This problem was caused by me having ADB installed twice on my machine. Once in my home directory (used by Android Studio), and again in /usr/bin/adb as I had the android-sdk package installed from aptitude. As Debian tends to be a bit slow with package updates, adb from the command line was calling the older version at /usr/bin/adb rather than the newer version in my home folder. This issue was solved by running:
sudo apt-get remove adb