When connecting to your Trados GroupShare server via HTTPS, you are receiving the following error:

There was a communications or connection error.
Please check that your server connection settings, network and internet connections, and that the server is available.



Expanding the error message has more information on the issue:

--> An error occurred while making the HTTP request to https://[ServerName]/platform/sdl/discovery.svc. This could be due to the fact that the server certificate is not configured properly with HTTP.SYS in the HTTPS case. This could also be caused by a mismatch of the security binding between the client and the server.
---> System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send.
---> System.IO.IOException: Unable to read data from the transport connection: An existing connection was forcibly closed by the remote host.



Full Trados Studio logs:

FATAL [Time Stamp] Sdl.Desktop.Platform.UnhandledException - There was a communications or connection error.
Please check your server connection settings, network and internet connections, and that the server is available.
System.ServiceModel.CommunicationException: An error occurred while making the HTTP request to https://[ServerName]/platform/sdl/discovery.svc. 
This could be due to the fact that the server certificate is not configured properly with HTTP.SYS in the HTTPS case. 
This could also be caused by a mismatch of the security binding between the client and the server. ---> System.Net.WebException: 
The underlying connection was closed: An unexpected error occurred on a send. ---> System.IO.IOException: Unable to read data from the transport 
connection: 
An existing connection was forcibly closed by the remote host. ---> System.Net.Sockets.SocketException: An existing connection was forcibly 
closed by the remote host
   at System.Net.Sockets.Socket.Receive(Byte[] buffer, Int32 offset, Int32 size, SocketFlags socketFlags)
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   --- End of inner exception stack trace ---
   at System.Net.Sockets.NetworkStream.Read(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.FixedSizeReader.ReadPacket(Byte[] buffer, Int32 offset, Int32 count)
   at System.Net.Security.SslState.StartReceiveBlob(Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.CheckCompletionBeforeNextReceive(ProtocolToken message, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.StartSendBlob(Byte[] incoming, Int32 count, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ForceAuthentication(Boolean receiveFirst, Byte[] buffer, AsyncProtocolRequest asyncRequest)
   at System.Net.Security.SslState.ProcessAuthentication(LazyAsyncResult lazyResult)
   at System.Net.TlsStream.CallProcessAuthentication(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean 
   preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Net.TlsStream.ProcessAuthentication(LazyAsyncResult result)
   at System.Net.TlsStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.PooledStream.Write(Byte[] buffer, Int32 offset, Int32 size)
   at System.Net.ConnectStream.WriteHeaders(Boolean async)
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.GetResponse()
   at System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
   --- End of inner exception stack trace ---

Server stack trace: 
   at System.ServiceModel.Channels.HttpChannelUtilities.ProcessGetResponseWebException(WebException webException, HttpWebRequest request, 
HttpAbortReason abortReason)
   at System.ServiceModel.Channels.HttpChannelFactory`1.HttpRequestChannel.HttpChannelRequest.WaitForReply(TimeSpan timeout)
   at System.ServiceModel.Channels.RequestChannel.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Dispatcher.RequestChannelBinder.Request(Message message, TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannel.Call(String action, Boolean oneway, ProxyOperationRuntime operation, Object[] ins, Object[] outs, 
TimeSpan timeout)
   at System.ServiceModel.Channels.ServiceChannelProxy.InvokeService(IMethodCallMessage methodCall, ProxyOperationRuntime operation)
   at System.ServiceModel.Channels.ServiceChannelProxy.Invoke(IMessage message)

Exception rethrown at [0]: 
   at System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage reqMsg, IMessage retMsg)
   at System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData& msgData, Int32 type)
   at Sdl.Enterprise2.Platform.Contracts.Discovery.IDiscoveryService.GetServiceEndpoints()
   at Sdl.Enterprise2.Studio.Platform.Client.Discovery.DiscoveryServiceClient.GetServiceEndpoints()
   at Sdl.Enterprise2.Studio.Platform.Client.IdentityModel.UserManagerClientFactory.GetUserManagerVersion(String baseAddress)
   at Sdl.Enterprise2.Studio.Platform.Client.IdentityModel.UserManagerClientFactory.Create(String baseAddress)
   at Sdl.Desktop.Platform.ServerConnectionPlugin.Implementation.ServersDialog.ServerConnectionWorker.Connect()
INFO [Time Stamp] Sdl.Desktop.Platform.UnhandledException - There was a communications or connection error.
Please check your server connection settings, network and internet connections, and that the server is available.

Solution 1
Connect to the Trados GroupShare server using Trados Studio 2019 or a newer version.
 

Solution 2

If it is not possible to use Trados Studio 2019 or a newer version, enable the TLS 1.0 protocol on the Trados GroupShare Application Server or connect Trados Studio 2017  over HTTP (through port 80 if open on the server).
 

Solution 3

If you do not want to enable the server TLS 1.0 protocol nor do you want to open port 80 for HTTP connection, apply on the local computer (user machine where Trados Studio is installed) the below registry value to enforce Trados Studio 2017 to connect through a TLS 1.2 protocol via HTTPS (port 443)

1. Close Trados Studio
2. Go to registries at HKEY_LOCAL_MACHINE\SOFTWARE\WOW6432Node\Microsoft\.NETFramework\v4.0.30319 
3. Add a new DWORD Value with Value Name SystemDefaultTlsVersions and Value data: 1

You can also apply the attached registry file to deploy the value automatically.
4. Open Trados Studio and go to File>Setup> Servers and attempt a new connection to the server via HTTPS (port 443)

Solution 4
This solution is likely applicable to Windows 7-running PCs

1. Close Trados Studio
2. Go to registries at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\ and add a New Key, under Protocols naming it TLS 1.2
3. Under the TLS 1.2 Key, create a New Key, naming it Client
4. The full registry key path should look like this now HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.2\Client
5. Inside the Client key, add a new DWORD Value with Value Name DisabledByDefault and Value data: 0 .