Thank you so much Kevin for pointing me in the right direction! I guess Google didn't return anything because of the changed error wording compared to the initial reports. Looking forward to the fix!

As of 26.1.0, the error has disappeared, however an attempt to create bookmarkData for "/" returns instead a bookmark for "/.nofollow/" (which is, obviously, pretty useless)

As of 26.1.0 the error has disappeared, however an attempt to create bookmarkData for "/" now results in a wrong bookmark for "/.nofollow/".

Unfortunately, using the .icon representation type on QLThumbnailGenerator is the only way to get the new "rich" folder icons on macOS 26.