Falls das was hilft:
Subsembly.FinTS.ᙒ: Socket Error ---> System.Net.Sockets.SocketException: Der angegebene Host ist unbekannt
at System.Net.Dns.GetAddrInfo(String name)
at System.Net.Dns.InternalGetHostByName(String hostName, Boolean includeIPv6)
at System.Net.Dns.GetHostEntry(String hostNameOrAddress)
at Subsembly.FinTS.FinCommServiceTcp._ᙏ(String Ꭽ)
--- End of inner exception stack trace ---
at Subsembly.FinTS.FinCommServiceTcp._ᙏ(String Ꭽ)
at Subsembly.FinTS.FinCommServiceTcp.ᘣ(String Ꭽ, String Ꭾ, ᘪ ᘤ)
at Subsembly.FinTS.ᙰ._ᚺ(ᛲ ᚙ)
at Subsembly.FinTS.ᙰ.ᚓ(ᛇ ᚔ, String ᚕ, String ᚖ, String ᚗ)
at Subsembly.FinTS.Online.᠃._ᠠ(ᛇ ᚔ, String ᚕ, String ᚖ)
at Subsembly.FinTS.Online.᠃._(String ᚕ, ᛇ ᚔ, ᜫ& )
Das steht in der Fehlermeldung drin.