Only certain file types are allowed in Lightroom. What file types are the photos that do not appear in Lightroom?
Most of my files are .raw (Panasonic FZ100) with corresponding .xmp. A lot of .jpg as well and even some .psd, .tiff and .avi.
I can not say that for some types all is exported or nothing. At least a lot of .jpg is exported - but only a small amount. I'm not sure - i will test later, when I'm at home - if also .raw is exported in some cases.
I tried to find out, if it is a matter of path/filenames. But I don't see any dependency. In fact, some pathes have german "Umlaute", a lot of pathnames have charachters like <space>, "-" or underscore. But even the folders PSE or Lightroom create, for instance as date in the name, there are <space> in the filepathes. So, I don't think, that this matters.
Is there any limit in Catalog-Size??? On volume, there is space enough. May be, some preferences have to be changed?
No catalog size limit.
All of those files should be in the upgraded Lightroom catalog.
I have seen occasional reports in this forum of this happening, where some (lots) of photos in PSE do not wind up in the upgraded LR catalog, but I am not aware of any solution.
The only suggestion I have is for you to try it again, and to search these forums for "umlaut" and also for other people who have had this issue.