My Roon Nucleus purchased in July '21 directly from Roon has been dropping connections for the last few weeks. Normally, a reboot sorted things out. This morning after a dropped connection I did the following:
reboot - afterward device could be connected to via web browser but not by the app
restart server - afterward device could NOT be connected to via web browser or by app
This device (a rev B) had its original m.2 SSD replaced under warranty in September '21.
Reporting back on this, @Vincent_Kennedy you nailed it. This particular network hardware was operating fine with other attached devices but I rebooted all and the Nucleus reconnected as per normal.