I have made a simple test program that repeatedly fires multiple concurrent HTTP POST requests against a server, StatHat.com in the example.
The program sends 30 simultaneous requests every 30 seconds and works great under Swift 5.0 under Linux and Mac, and fails under Swift 5.1 Linux. Works under Swift 5.1 Mac.
The expected output is that all https requests finish within several seconds with HTTP status code 204. Under Linux 5.1 couple of them do finish but rest starts timing out with error code 1001.