I found the culprit! The whole synchronisation task was polluted by one single directory name that contains a combination of characters that SMB does not like, when it is perfectly accepted on the Syno.
Look at this directory name:
Les Siècles, François-Xavier Roth - Berlioz - Harold en Italie, Les Nuits d’été (2019)[FLAC-24bit-441000Hz]
In that form, it works, at least you can see it and open it from the Finder, and it can directly be put in the active playlist.
Now, look at this:
Les Siècles, François-Xavier Roth - Berlioz - Harold en Italie, Les Nuits d’été (2019) [FLAC-24bit-441000Hz].
Here, it does not work because of this space before the brace!! Interesting, isn’t it?
Yet another mystery of the SMB thing, another piece of excellence by MS!!
Now, I would need to write a script that could detect all those things before trying to put them into the Audirvana library.
But, what is strange is the effect such file has to synchronisation algorithm. Instead of being blocked when that file is met, it stops a bit later, or it can even continue but without processing all the files.
The only error I got was that the file did not exist as it was searched with a name that did not match the name reported by the SMB protocol, due to that combination of space and brace.