Thanks for the additional information, @Arie_Bos!
In reviewing your responses here, I just wanted to clear up what may be a point of confusion. Based on your report, I do not believe that your data base was deleted as you mentioned above.
While you may not be seeing your library, you should see that your playlists and tags still exist Roon, and those come from your Roon database. It sounds like the problem here is the connection from your Synology, where your library files are stored, to your Roon Core machine.
If I am not mistaken the HP Procurve 1810 G24 is a managed switch.
I usually like to mention that we’ve tracked a number of issues to in the past to managed switches(examples: here, here, and here).
The simplest way to confirm whether the switch (or its settings) are contributing to these symptoms would be to temporarily test with the switch removed from your network. Whether things improve or not, the results of that test would be a great data point. I know you mentioned it’s not ideal to perform this test, but it would certainly help with narrowing down the possible root cause here.
I would also recommend enabling flow control on the switch, as we have seen that settings resolve a number of issues for other users in the past.
Regards,
Dylan