Rooting my i9515 was smooth using CF-autoroot (http autoroot.chainfire.eu).
I had the same rebooting problems. Symptoms: A short vibration, the Samsung logo showing like the normal boot sequence and after that all apps were killed/restarted. In other words, anything a full reboot would do. It happened several times per day, sometimes when using the phone, sometimes when it was just lying on my desk. It could even happen at the moment when I picked up an incoming call. It wasn't a very usuful phone like that.
Uninstalling all apps with "knox" in the name (about 6) and also "KLMS agent" which had the same icon, using Titanium backup Pro, seems to have resolved the issue. I have not had a spontaneous reboot since, for about a week now.
Thanks all for reporting; I wasn't sure whether the reboots were caused by rooting, but since multiple people reported the same symptoms it was quite clear.
Now just waiting for a proper recovery and ROM to arrive for this model (preferrably Cyanogenmod). From what I've seen reported, all ROMs up to now fail to make the touchscreen working (which is worse than constant reboots ).