To give a little more information to my earlier post...
I only use MC for music, nothing else. I am accessing the music server using screen sharing. When I open MC19 it appears fine until I try to get it to show either of the libraries. It reports that it is loading and that I should please wait but then it hangs and the cursor turns into an eraser; wiping blocks of information out as it tracks over the window. I can only then force quit.
In the Library/Application Support/J River folder I have distinct Media Centre 18 and 19 folders but their contents differ. In MC18 (which still works fine) I have: Cache, Cover Art, Default RAW Image Cache, Library, Log.txt, Second library, Settings, Temp and Thumbnails. In MC19 I have: Default RAW Image Cache, Library, Log.txt, Previous Log.txt, Settings, Temp and Thumbnails. Although MC19 reported having restored the Second library it did not appear in the MC19 folder. I tried copying it across manually, fully aware that this would probably not work and, sure enough, it didn't. Having two, distinct, libraries is very important to me and how the music is managed so I would like to find a way of sorting this out.