Upgrade from SEP 12.1 to SEP 12.2 impossible: keeps asking for reboot

I am trying tu upgrade from SEP 12.1 to SEP 12.2 on a Windows SBS 2011 server. The installer keeps nagging me with the following remark:

"Symantec Endpoint Protection Manager has detected that there are pending system changes that require a reboot." and then asks me to reboot the machine. The messages comes back, even after reboots.

I read there are a number of solutions to similar problems when doing upgrades to 12.1. Can someone please tell me how to correctly handle this in my scenario with the never version?

Kind regards,

Wouter Pinkhof