So, in the absence of any answer, I reply to my own post.
First, I discovered I had some network issues: network connection between my Mac and the NAS was very bad for unclear reasons. I restarted all switches and the NAS and things improved.
I restarted Audirvana and the library was updated. Then I checked the DB integrity and rebuild the index and now it seems OK.
But, what is quite strange is the time it takes to update the DB after only one single addition of an album. And during that process, that may take a while to complete, I noticed the beachball spinning quite often