Well first of all, the message that the display isn't attached to the NVIDIA GPU is accurate.  The display outputs in that system aren't wired directly to the NVIDIA GPU.  They're wired to the Intel GPU.  This is the setup used by the vast majority of laptops that have both Intel and discrete GPUs.  In this setup, the discrete GPU when needed operates as a render-only device that passes completed video frames to the Intel GPU for output to the display(s). On the NVIDIA side, this technology is called NVIDIA Optimus. The main reason for this design is to improve battery life, because this allows the discrete GPU to be completely powered off when it isn't needed, whereas if the discrete GPU was directly wired to display outputs, it would need to remain active whenever a display was attached, even if nothing graphics-intensive was going on. But the fact that the NVIDIA GPU doesn't directly control the display output is why certain controls in NVIDIA Control Panel aren't usable.  Another downside to this design is that there are certain technologies that Intel GPUs don't currently support passing through and/or that require the discrete GPU to have direct control of the display output in order to be used, such as VR, G-Sync, 5K resolution, stereoscopic 3D, Adaptive V-Sync, and possibly others.

However, I'm not sure what would have changed that would cause this to start happening all of a sudden, because the display output wiring is a hardware design aspect of the system that wouldn't have changed.  And if you can't use the dGPU's acceleration at all on the external displays, I'd want to know more about the dock you're using.  It's a known issue that docks that use DisplayLink chips for handling video cannot use dGPU acceleration in dual GPU systems.  That's due to a Windows limitation that causes only the primary GPU to be usable for handling DisplayLink traffic, and the Intel GPU is mostly if not always primary in such systems, but a Thunderbolt dock shouldn't be using DisplayLink technology.

I forgot to mention that especially since you're using an XPS 15, you might want to look at a Dell Thunderbolt dock, since they're the only ones that will properly charge an XPS 15.  The reason is that the XPS 15 models require a 130W power source for proper functionality, but the official USB Power Delivery spec only goes to 100W, and many docks don't even provide that much power.  As a result, if you use third-party docks with an XPS 15, you either need to attach the system's AC adapter separately or else put up with potential performance throttling under load if the dock is supplying less than 130W, because in that case the system will try to reduce its power consumption to operate within the lower available power budget.  If you use Dell docks, Dell did something proprietary to allow some of their USB-C and Thunderbolt docks to provide up to 130W, specifically to support systems like the XPS 15.  If you ever want to go that route, your best dock options would be either the new WD19TB or its predecessor the TB16.  If you go with the TB16, make sure you get the version that comes with a 240W adapter, NOT the version with a 180W adapter, because the latter can't provide 130W to the attached system.  The WD19TB only comes with a 180W adapter, but that model can provide 130W to the attached system even when the dock itself is running from a 180W adapter.

I agree with everything but it did work super good for a year now and it just stopped. Do you have an idea if I can load an old Bios version to see if that fixes the issue? And if yes I was not able to find the old versions somewhere, do you have an idea where I can get them?

Thx but I have already a dock and like to keep it and not  buy a new one now