TFS 2018 on premise behind proxy service hooks notification failure - service

Question

since few days, service hooks all fail to notificate (PR updates, Work Item comments and so on) with this error and I don't know why.
TFS is installed on a domain machine (active directory) and had no problems so far.
The funny thing is that if I test in the Service Hook it does work
Slack or Teams. Doesn't matter.
Any ideas?
Error Details
System.Net.Http.HttpRequestException: An error occurred while sending the
request. ---> System.Net.WebException: Unable to connect to the remote server
---> System.Net.Sockets.SocketException: A connection attempt failed because
the connected party did not properly respond after a period of time, or
established connection failed because connected host has failed to respond
143.204.9.158:443
at System.Net.Sockets.Socket.InternalEndConnect(IAsyncResult asyncResult)
at System.Net.Sockets.Socket.EndConnect(IAsyncResult asyncResult)
at System.Net.ServicePoint.ConnectSocketInternal(Boolean connectFailure,
Socket s4, Socket s6, Socket& socket, IPAddress& address, ConnectSocketState
state, IAsyncResult asyncResult, Exception& exception)
--- End of inner exception stack trace ---
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.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter
.HandleNonSuccessAndDebuggerNotification(Task task)
at Microsoft.VisualStudio.Services.ServiceHooks.Common.HttpActionTask.
<RunAsync>d__8.MoveNext()