There are a number of features in SketchUp 2026 which aren’t available in SketchUp 2024. It occurred to me that in some of your models you might have included Environments and Photo Real textures which would add greatly to file size in 2026 but you would lose those in 2024.
I don’t know how large a file can be copied into the “pasteboard” on Mac. I had no issues copying the viewports and pasting them as viewports but I’m working with Windows 11 and there may be a difference in max file size between the two platforms.
If environments and PBR materials are adding enough to the file size to exceed that pasteboard limit I would expect some issues. Definitely purging unused environments and materials would reduce things. Might need to eliminate the environment images altogether.