Unrooted Samsung Galaxy S5 running Lollipop 5.0 randomly reboots

Since I installed Lollipop on my Galaxy S5, my phone is trying to reboot itself randomly approximately once a week. But it gets stuck on the boot-up logo screen then, only showing the word SAMSUNG there. If I don't do anything about it, the display is lit up with the logo, until the battery is completely emptied (the battery-graph will show a gap for that time later, not telling me what could have happened). This is especially very annoying at night, since I use my phone as an alarm-clock ;) Does anyone have suggestions about it? Anything to disable that may cause these spontaneous reboot-attempts? * The phone is not branded * I did not root it * I already tried to 'factory-reset' my device * Everything worked fine prior the Lollipop-Upgrade