Connecting...connection failed

Using my ipad pro with ios 14.4.2, to connect to my Roon server, I find frequently the connection is not possible. When this happens I can see the client is trying to connect but the following error shows again and again: “connecting…connection failed”.
The PC running the server is working and I am able to manage locally the Roon core software without any issue. Also I can connect to the same server from my ipad using any remote desktop app, so I understand the network is working.
This problem happens by days, some days I can work with Roon without any trouble and some days it refuses to connect.
The core is connected through Ethernet to the router. The ipad connects by wiFi to this router.