Loss of resources

Hello all, this is my first post and a cry for help.

I am using Joplin only a week or so on 2 PC's with a sync on my personal server (Synology).
All data was imported from Evernote that I used almost 10 years.
Yesterday I discovered that the resources on one of the machines lost almost all of the 1737 files.
After browsing this forum I checked the sync server location but that seems OK. So I copied the resource dir from the other PC, stopped Joplin on the troubled PC and pasted the copied data. Now all seems to be alright.
Still wondering what possibly has gone wrong.
Any suggestion would be appreciated.