The problem has already been described by cchapeline here and on "Blender Stackexchange" here (but without any fitting solution for my problem). Despite exactly following Kent's @theluthier instruction, the columns nevertheless turn magenta as soon as the second "Prinicpled BSDF" is mixed in with a "Mix Shader" node:
After muting the "Mix Shader Node":
With just the "Principled BSDF" with the paint color beeing plugged into the "Material Output" node:
Everything fine in Cycles:
All tests run with Blender 2.92.0 and 2.93 Alpha.
Thank you, @adrian2301 😀! Yes, I'm using Windows which is updating very often in the last time. It's a little bit annoying but on the other hand I don't want to miss any security gap fixes.
I do my updates every couple of weeks. When an update is released there is normally an update to the update a week to 10 days later to resolve any issue reported by the original update, so your not really missing anything. And I get to do it when I'm not Blendering and slowing down my computer. With the GPU you are best checking manually because Windows updates will only give you basic drivers. My GPU is an AMD, I have to update the standard drivers plus install pro drivers to use Blender.
@adrian2301 and 
spikeyxxx Blender on my computer is running on GPU. It's a gaming laptop with Blender as my favorite game 😉! So, one should think that there won't be such problems. I'm wondering what monster 👹 graphics card Kent @theluthier is using 🤔?
duerer I've got a GeForce RTX 2080 (8GB VRAM). It's not new but it's on the higher end still...maybe?
Thank you, @theluthier! I'm wondering whether part of the problem could be an outdated driver. When I'm starting the program for the system settings of my GeForce GTX 1060 the program crashes after a few seconds before I can change anything:
I need to search for more informations on how to fix this isssue. The problem with updated drivers is unfortunately that they sometimes make things even worse. I've read so often about such problems which I've already experienced myself in the past but not with my current computer.