Similar thread to this one.
None of the answers were helpful.
- It's an unrooted Android 5.1.
The device has been factory reset.
- The phone does not overheat, so the encryption clearly hasn't begun.
- I'm able to cancel the process by holding power button for 10s.
Once rebooted, the system is untouched, just as it was before the encryption attempt.
Retried the process with
$ adb logcatactive on USB.
Nothing suspicious in the logs, although I don't know what to look for too.
Meta: Will delete the link with logs and paste only relevant lines once there's a working solution.