The request was aborted: Could not create SSL/TLS secure channel

[3CXusername]

Server running: Pro v15.0.62928.5 (Windows Server 2008 x64 R2)
Clients running: 15.0.62928.52 (all local) / WRSA A/V

This was not an issues previously with v14 sp3 on same system. Since upgraded system to v15 sp5 clients receiving:

Client log shows:
[10.04.2017 12:32:38.144][1] Error event caught, Cannot send request: RequestChangeStatus, channel [ 61909621 at https://local.3cxserver.com/myphone/MPWebService.asmx ]
[10.04.2017 12:32:38.149][1] System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.WebException: The request was aborted: Could not create SSL/TLS secure channel.
at System.Net.HttpWebRequest.EndGetRequestStream(IAsyncResult asyncResult, TransportContext& context)
at System.Net.Http.HttpClientHandler.GetRequestStreamCallback(IAsyncResult ar)
--- End of inner exception stack trace ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at MyPhoneClientChannelNet.Implementation.ClientChannelImpl.<doRequestAsync>d__57.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at MyPhoneClientChannelNet.Implementation.ClientChannelImpl.<RequestAsync

I have reinstalled the clients from scratch and it comes up once/twice a day. All .nets are installed.

Anyone run into this?

Thanks in advance