adb – Nexus 9 stuck in reboot loop

Few days ago I started my Nexus 9 and it didn’t manage to load. It started looping.

The loop goes like this:

The four colored dots circling around and then loops again – 5-6 times then it shuts down.

I haven’t done anything with it – it’s unrooted, original firmware, used it normally since the day I bought it (have it for about 5 years).

I tried anything I knew:

-factory settings,
-wiped cache,
-flashed original (newest firmware),
-flashed earlier versions,
-flashed original HTC firmware,
-sideloaded newest firmware,
-sideloaded an earlier version.

Every procedure ended with a success, but the result was always the same – four colored dots circling around and then loop again …

Any suggestions (besides “get a new tablet”)?