Thanks Dr. Sassi
Found it.

I am looking for a jpg file stored in my assets.
the path starts with assetdb:/// but the image can't be located on my disc.
How can I access the file?

After intense mail exchange with tech support they came to the conclusion it is a MacOS Sequoia thing.
It’s in the developers hand now.

It must be a 2025 thing. I opened the same model in 2023 and it all runs smoothly.
I changed all preference settings to the same in 2025 but no change at all.

Tech support came with these solutions but none did work.
I am desperate.

Regarding this issue, there are some ideas you can try:

We appear to have found an issue with the Apple Shader Compiler that is causing a problem with the current Redshift.
To prevent this, you need to delete older Redshift caches.

Quit out of Cinema 4D!
Open on your hard disk the folder "/Users/username/redshift" and delete there the "cache" folder.
Re-Start Cinema 4D and the cache will be newly compiled.

Even using the new Asset Browser extensively might slow down the program. There are some ways to speed up the Asset Browser a bit, following these ideas:

Using databases, with Cinema 4D S26 we added the possibility to index a database.
This would speedup the startup of Cinema 4Ds Asset Browser and using these assets.
You just need to rightclick your database and select "Create Index":

This procedure might take a few minutes once, but afterwards issues with long loading times should be reduced enormously.

In case of having Watch Folders added, assure to not have to huge folders, as these folders are cached and loaded at startup and the larger they are, the longer startup of the program will take. Unfortunately, this is a limitation of these watchfolders and without beeing so limited, it would be that simple and just giving the possibility to browse directly.
In general Watchfolders have been added as possibility to browse parts of your harddisc (special folders with scenes or assets) temporarily without the need of creating a database.

Watchfolders are slower but more easy to handle. We will further on provide both ways and it's up to you, to decide what way you prefer.

Switching another option for Material Previews might help too:

I have a navigating issue. When I try to move, scale or orbit the camera in my project the viewport isn't reacting immediately but chunky when my materials are turned on. (options panel, material N-Q). As soon as I turn them off the viewport is smooth again.

@Dr-Sassi I didn't used the Mo-Spline yet. Have to dive into it sometime.
But I solved it by using a single spline with a Sweep.

How can I connect 2 polygons in a way 2 splines are connected with a loft.
The loft isn't very accurate when I use the edge to spline command.

Even in small scenes my viewport is choppy when materials are on in viewport options.
When i try to rotate the viewport it becomes very choppy in quick shading.
What could possibly be wrong?
These are my Viewport Hardware settings.

Wow. Thanks a lot.