Thank you for your reply !
Unfortunately, thats my ISP router and I can't afford to buy a new one for now, to get a better WiFi performance/stability. It's not crashing every days, but it's quite annoying to unplug the device every time.
This "bug" always bothered me and I though there was a fix (or an issue with my configuration) because VLC or Windows doesn't crash when the ethernet/WiFi is down.
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edit : is there a NFS timeout option ? A 30s timeout or something like that should be fine.