Hello everyone!
I'm sorry for my late reply, I have been on a extended vacation during the Easter holidays!
@elwislak, very sorry to hear about this. 
I can see from your newest post that device is not starting up at all now, have you been in contact with your Local support team and they have arranged a repair for you yet? If you haven't done that, the last thing you could try is performing a software repair using Xperia Companion. This can be done without the computer detecting the device or if you aren't able to turn the device on. However if you experience the same behaviour after the software repair has been completed, your device will need to be examined by a trained technician at one of our authorized service centres.
@schetz, it is very hard to troubleshoot something that does not occur that often and having your device rebooted in safe mode for a longer period of time is not the most fun thing ever. You mentioned that this occurred while on the concept software as well and if I remember correctly you had to perform a software repair to get rid of this software and go back on the "normal" software right?
Do you recall if you are doing something specific when the reboot occurs, for example is it during a call or when using a specific application or service?
@Handmann, sorry to hear about your device.
Your question has been translated to English as this is an International forum.
Have you had the chance to take a look at my previous troubleshooting that I have posted in this thread? If you haven't done that, I would recommend starting with restarting your device in safe mode!
Keep me posted!
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