Minor logging change, and upgrade package. I've tested in 2021.2.0b11 that this works with DTLS active, although it might need a little more rigor since I had intermittent issues with one editor (but not so much the other?) failing to fully connect to Relay for some reason (i.e. getting stuck in the "Connecting..." state).

Assets/Scripts/Relay/RelayUtpClient.cs

                 MsgType msgType = (MsgType)msgContents[0];
                 int idLength = msgContents[1];
                 if (msgContents.Count < idLength + 2)
+                {   UnityEngine.Debug.LogWarning($"Relay client processed message of length {idLength}, but contents were of length {msgContents.Count}.");
+                }
 
                 string id = System.Text.Encoding.UTF8.GetString(msgContents.GetRange(2, idLength).ToArray());
                 if (id == m_localUser.ID || !m_localLobby.LobbyUsers.ContainsKey(id)) // We don't need to hold onto messages if the ID is absent; users are initialized before they send events.

Packages/manifest.json

     "com.unity.test-framework": "1.1.27",
     "com.unity.textmeshpro": "3.0.6",
     "com.unity.toolchain.win-x86_64-linux-x86_64": "0.1.20-preview",
-    "com.unity.transport": "1.0.0-pre.3",
+    "com.unity.transport": "1.0.0-pre.5",
     "com.unity.ugui": "1.0.0",
     "com.unity.modules.ai": "1.0.0",
     "com.unity.modules.androidjni": "1.0.0",
     "com.unity.modules.xr": "1.0.0"
   },
   "scopedRegistries": []
-    }
+}

Packages/packages-lock.json

       "url": "https://packages.unity.com"
     },
     "com.unity.transport": {
-      "version": "1.0.0-pre.3",
+      "version": "1.0.0-pre.5",
       "depth": 0,
       "source": "registry",
       "dependencies": {