Hi everyone,
first of all thanks a lot Dr. Sassi, you're doing a great job here but it sometimes seems unfortunately you are the only one 
I think I can clear things up a bit, but it only makes it seem worse considering the origin of this problem.
AFAIK – means I read it on the interwebs – Apple changed the way thumbnails (and quicklook) works going forward from Sequoia … check this website for details:
https://eclecticlight.co/2024/10/31/how-sequoia-has-changed-quicklook-and-its-thumbnails/
so if we take a look in the package of the most recent cinema4d update from april 25 (sic!) we find this fella here: "Cinema 4D.qlgenerator"
Let's quote the article again:
"...To check whether an app is still trying to use an old QuickLook Generator, look inside the app bundle in Contents/Library/QuickLook. If you see one or more .qlgenerator bundles there, then those no longer work in Sequoia."
So the root of the problem is that it's not a bug, it's simply Maxon failing to implement changes in macOS that have been announced years ago and now are live and breaking things in Cinema4D since October last year at least. That would be bad enough, but I alone already filed two support tickets in this time which have both been marked "solved" not even tackling the real problem, and I guess many others have done the same and/or are fighting with the problem. And Maxon recommends using the latest versions of macos since apple fixes gpu drivers all the time, so seriously what the hell is going on?
BTW Anchorpoint, a 3rd party tool somehow manages to display thumbnails for C4D on sequoia, maybe Maxon should ask Matthäus to come back and fix a few things in the code 
I just got the email that my MaxonOne Sub is coming up for payment again, and I never felt so bad about spending the money for C4D since I started using R13 long long time ago.
Oh hey, the article sums it up pretty nicely btw:
"If the app you rely on to generate custom QuickLook Thumbnails and Previews doesn’t yet come with those App Extensions, contact their Support and ask them when they’re going to implement the changes brought five years ago in Catalina. Particularly if you’re paying them a subscription, it’s time they caught up. Until they do, I’m afraid those Thumbnails and Previews simply won’t work in Sequoia, and you’ll continue to see generic icons rather than Thumbnails."
Best, Matthias
EDIT: Sorry, just noticed the website already has been posted, which makes me feel even more disappointed now how little progress has been made.