Core Plus reboots itself after system update

[Wojtek_wk]

Hello, I have a problem with my Samsung Galaxy Core Plus (I am writing about this issue here since I haven't found any section related to Galaxy Core Plus). Some days ago my phone told me that there was a system update available, so I performed it. Everything was OK but now, sometimes, about 5 times a week, my phone reboots itself with no reason. It happens when I'm not using it, when it is locked. When it happens, I want to unlock my phone and it requests to enter PIN number after reboot... What could be the reason and how could I solve it? Thank you very much for your help!