Not for the first time I encountered the problem, that Adobe Lightroom is a bit picky about file types and file type internals. Especially tiff-files are trial and error. To compute panorama images I use Hugin, which is a great software by the way. But unfortunately, I can't import the resulting 16bit tiff-files into Lightroom.
I already figured out that tiff-files saved by Gimp could be imported, but Gimp does not support 16bit files. Next I tried Blender, my beloved open source software of choice for 3D stuff and any weirdo image manipulation if nothing else helps. Initial success was soon impaired by the fact, that it kept crashing with images 30 megapixels / 350MB.
Thus I started searching for other solutions and finally found an old installation of fdrtools on my machine. Image > Open ... at least it opens the image and offers various tiff-settings while saving it again. And there we go. 16bit uncompressed tiff, saved without any problems and opens like a charm in Lightroom.
Some finishing touches et voilà:
Das Schloss Montfort in Langenargen