I managed to fix the problem myself by using the Support Assist OS Recovery.
I have purchased a Dell Precision 7760 from an official Microsoft Refurbisher. The device was sold as "new", however the problem is, that it won't power off after shutdown. In the meantime I have...   The issue remains the same; the device shuts down, but Power LED and/fans stay active and keyboard lights up when keys are pressed. The notebook can't be started and only a forced shutdown can power it off. I'm aware of that disconnecting the bios battery might be a possibility, but as this would mean to disassemble the 7760, I'd rather keep my hands off this. However, I've wondered if there is anything else that can be done? Any help appreciated. THX in advance!
Precision 7760 shuts down but doesn't power off