my phone has been at services twice already.
Firstly was Samsung official repair service, I posted phone in secure bag with a message explaining what happened, but after two weeks I got it back with notice "it is rooted, no warranty repair". They said, could do it for £150!
Secondly I found other official repair service, where you can bring the phone in person. That is Geek Squad in Carphone Warehouse stores, I found one in Glasgow, Scotland. The guys were very pleased to help, and promised that though the warranty is voided they are going to repair it.
They were going to fix it in three days, but unfortunately they couldn't find what cause the phone freeze up. What they done is sent it to their repair centre near Birmingham, but there phone was refused to be repaired because the same reason - warranty void. When I came to collect the phone in Glasgow on Saturday 8th of March I was able to speak to one of this guys and he explained me they can't make phone working properly. Even while talking with him phone was not responding sometimes and we had to take the battery off a few times. He gave the phone to other engineer to look up again. He opened it and stated that he didn't see any damage in the phone and couldn't actually find the fault, it seemed like the phone should work fine, but it is not. They have flashed my phone with Official Samsung's software (they couldn't change the Knox flag to 0x0) and sent me with it to the seller (Phones4U).
I have taken my phone home and the device was working fine for about two hours, it does that when is not so warm. I only wanted to check is it going to freeze up again. So I played some video to heat up the phone and it stopped responding again. I took off battery and left it overnight.
Next day afternoon I pick up my phone, power it on, and I was trying to check is it possible to update the software when it is official one (that might solve the problem). I could only see the message saying your device is modified, cannot update. I checked it in download mode, and the phone was back from official to custom software!
There was no my contribution in it. The phone did it itself.
So I can conclude that if phone is changing software status from official to custom when this fault comes out, it could also change the status of Knox Warranty Void from 0x0 to 0x1. If that is true, I will have a long way to prove that. But now I'm sure that no person is responsible for tripping the flag in this phone, that could be done by the phone itself.
That is very weird! That means Samsung is putting some extra security (KNOX) to their devices which can cause the phone to be out of warranty when some fault like overheating exists!