Socket read operation has timed out after 30000 milliseconds - after several successful connections

Hi!
I have a different issue then similiar ones. After serveral successful connection I'm getting an error as in the title.
It's a Windows Service that checks documents status on a SFTP server. And, yes, it is a proper SFTP (SSH) server. I think it's an ISP issue on server side that adds all IPs to blacklist, but I have to make sure that this is not a bug...

My application connects to SFTP server every 5 minutes. Tried on 3 different machines with different IPs, everything ends with the same error after couple successful ones.

Scheduler is Quartz.

2020-03-13 16:05:00.8389|INFO|CONNECTED
2020-03-13 16:10:00.7614|INFO|CONNECTED
2020-03-13 16:15:00.8870|INFO|CONNECTED
2020-03-13 16:20:00.9344|INFO|CONNECTED
2020-03-13 16:25:00.7007|INFO|CONNECTED
2020-03-13 16:30:00.7013|INFO|CONNECTED
2020-03-13 16:35:00.7175|INFO|CONNECTED
2020-03-13 16:40:00.7181|INFO|CONNECTED
2020-03-13 16:45:00.7187|INFO|CONNECTED
2020-03-13 16:50:00.7037|INFO|CONNECTED
2020-03-13 16:55:00.6886|INFO|CONNECTED
2020-03-13 17:00:00.7517|INFO|CONNECTED
2020-03-13 17:05:00.6898|INFO|CONNECTED
2020-03-13 17:10:00.7061|INFO|CONNECTED
2020-03-13 17:15:00.6754|INFO|CONNECTED
2020-03-13 17:20:00.7854|INFO|CONNECTED
2020-03-13 17:25:00.6922|INFO|CONNECTED
2020-03-13 17:30:00.6928|INFO|CONNECTED
2020-03-13 17:35:00.6934|INFO|CONNECTED
2020-03-13 17:40:00.6940|INFO|CONNECTED
2020-03-13 17:45:30.5072|ERROR|Renci.SshNet.Common.SshOperationTimeoutException: Socket read operation has timed out after 30000 milliseconds.
   w Renci.SshNet.Abstractions.SocketAbstraction.Read(Socket socket, Byte[] buffer, Int32 offset, Int32 size, TimeSpan timeout)
   w Renci.SshNet.Session.SocketReadLine(TimeSpan timeout)
   w Renci.SshNet.Session.Connect()
   w Renci.SshNet.BaseClient.Connect()
   w OptimoKurierService.Jobs.MainJob.<Execute>d__0.MoveNext()
2020-03-13 17:50:30.5078|ERROR|Renci.SshNet.Common.SshOperationTimeoutException: Socket read operation has timed out after 30000 milliseconds.
   w Renci.SshNet.Abstractions.SocketAbstraction.Read(Socket socket, Byte[] buffer, Int32 offset, Int32 size, TimeSpan timeout)
   w Renci.SshNet.Session.SocketReadLine(TimeSpan timeout)
   w Renci.SshNet.Session.Connect()
   w Renci.SshNet.BaseClient.Connect()
   w OptimoKurierService.Jobs.MainJob.<Execute>d__0.MoveNext()
2020-03-13 17:55:30.5240|ERROR|Renci.SshNet.Common.SshOperationTimeoutException: Socket read operation has timed out after 30000 milliseconds.
   w Renci.SshNet.Abstractions.SocketAbstraction.Read(Socket socket, Byte[] buffer, Int32 offset, Int32 size, TimeSpan timeout)
   w Renci.SshNet.Session.SocketReadLine(TimeSpan timeout)
   w Renci.SshNet.Session.Connect()
   w Renci.SshNet.BaseClient.Connect()
   w OptimoKurierService.Jobs.MainJob.<Execute>d__0.MoveNext()
2020-03-13 18:00:30.5246|ERROR|Renci.SshNet.Common.SshOperationTimeoutException: Socket read operation has timed out after 30000 milliseconds.
   w Renci.SshNet.Abstractions.SocketAbstraction.Read(Socket socket, Byte[] buffer, Int32 offset, Int32 size, TimeSpan timeout)
   w Renci.SshNet.Session.SocketReadLine(TimeSpan timeout)
   w Renci.SshNet.Session.Connect()
   w Renci.SshNet.BaseClient.Connect()
   w OptimoKurierService.Jobs.MainJob.<Execute>d__0.MoveNext()
2020-03-13 18:05:30.5096|ERROR|Renci.SshNet.Common.SshOperationTimeoutException: Socket read operation has timed out after 30000 milliseconds.
   w Renci.SshNet.Abstractions.SocketAbstraction.Read(Socket socket, Byte[] buffer, Int32 offset, Int32 size, TimeSpan timeout)
   w Renci.SshNet.Session.SocketReadLine(TimeSpan timeout)
   w Renci.SshNet.Session.Connect()
   w Renci.SshNet.BaseClient.Connect()
   w OptimoKurierService.Jobs.MainJob.<Execute>d__0.MoveNext()

Beginne mit dem gemeldeten Stacktrace über Renci.SshNet.Abstractions.SocketAbstraction.Read, Session.Connect und BaseClient.Connect und überprüfe anschließend die wiederholte Verbindungssequenz aus OptimoKurierService.Jobs.MainJob. Reproduziere die erfolgreichen Verbindungen, auf die Zeitüberschreitungen folgen, und vergleiche das Verhalten der Bibliothek mit dem SFTP-Server und den Netzwerkbedingungen. Als erledigt gilt die Feststellung, ob die Zeitüberschreitung ein Fehler der Bibliothek oder ein externes Verbindungsproblem ist.