Nexus 10 not booting, showing Google and lock icon when turned on
So if the lock icon was closed before and just opened somehow, you should find out what happens.

Now the lock is opened, so your boot-loader is opened. Have you opened it?

There are BL-unlocking apps available but all of them need root permission so I think this is not the cause. But I'm sure the device did not open its BL.


  I lost access to it, and lost all my data. This made me not wanting to try Apple again. I seriously hope I'm not dealing with something like this here now again...


On the Nexus devices if you open the device's Bootloader all data will be deleted. It's a security concern. The Android documentation doesn't provide a lot of elaboration, stating simply:


  The procedure must be confirmed on-screen, and deletes the user data
  for privacy reasons.


Once the bootloader is unlocked, the rooting process is quite easy so all of the information on /data essentially becomes accessible. This could be emails, contact information, or many other pieces of personal information that you likely don't want in the wrong hands. Therefore, it makes sense to wipe all of this data during the unlocking procedure.

TL;DR: If the device bootloader was opened somehow, all of your data will be gone.