Nexus 9 stuck in endless reboot loop after trying to root it
Using Firelords' suggestion I was able to unbrick my device and get it working again.


Downloaded the Lollipop 5.1.1 image for Nexus 9 on this site: https://developers.google.com/android/nexus/images
Unpacked all the files into the folder where I had fastboot.exe.
Started my tablet by holding down the power button and the volue up botton to get into some kind of recovery mode.
fastboot-all.bat did not work for me, so ran the following commands instead:

fastboot.exe flash bootloader bootloader-flounder-3.44.1.0123.img
fastboot.exe flash system system.img
fastboot.exe flash recovery recovery.img
fastboot.exe flash boot boot.img
fastboot.exe flash cache cache.img
fastboot.exe flash vendor vendor.img

Reboot the device using the little menu on the screen. It was now able to start up, and I could recover from my latest backup.