Managed to get it to fail in debug mode. It is stuck on line 1499 in SFTPClient.cs.
Here is the debug message trace
SshNet.Logging Verbose: 1 : Initiating connect to 'XXX.XXX.XXXX:22'.
SshNet.Logging Verbose: 1 : Server version '2.0' on '9.99 sshlib: 7.0.0.0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'KeyExchangeInitMessage': 'SSH_MSG_KEXINIT'.
'SFTPTest.vshost.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Dynamic\v4.0_4.0.0.0__b03f5f7f11d50a3a\System.Dynamic.dll', Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
'SFTPTest.vshost.exe' (Managed (v4.0.30319)): Loaded 'Anonymously Hosted DynamicMethods Assembly'
SshNet.Logging Verbose: 1 : SendMessage to server 'KeyExchangeInitMessage': 'SSH_MSG_KEXINIT'.
SshNet.Logging Verbose: 1 : SendMessage to server 'KeyExchangeDhGroupExchangeRequest': 'SSH_MSG_KEX_DH_GEX_REQUEST'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'KeyExchangeDhGroupExchangeGroup': 'SSH_MSG_KEX_DH_GEX_GROUP'.
SshNet.Logging Verbose: 1 : SendMessage to server 'KeyExchangeDhGroupExchangeInit': 'SSH_MSG_KEX_DH_GEX_INIT'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'KeyExchangeDhGroupExchangeReply': 'SSH_MSG_KEX_DH_GEX_REPLY'.
SshNet.Logging Verbose: 1 : SendMessage to server 'NewKeysMessage': 'SSH_MSG_NEWKEYS'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'NewKeysMessage': 'SSH_MSG_NEWKEYS'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ServiceRequestMessage': 'SSH_MSG_SERVICE_REQUEST'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ServiceAcceptMessage': 'SSH_MSG_SERVICE_ACCEPT'.
SshNet.Logging Verbose: 1 : SendMessage to server 'RequestMessageNone': 'SSH_MSG_USERAUTH_REQUEST'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'FailureMessage': 'SSH_MSG_USERAUTH_FAILURE'.
SshNet.Logging Verbose: 1 : SendMessage to server 'RequestMessagePassword': 'SSH_MSG_USERAUTH_REQUEST'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'SuccessMessage': 'SSH_MSG_USERAUTH_SUCCESS'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelOpenMessage': 'SSH_MSG_CHANNEL_OPEN : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelOpenConfirmationMessage': 'SSH_MSG_CHANNEL_OPEN_CONFIRMATION : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelRequestMessage': 'SSH_MSG_CHANNEL_REQUEST : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelSuccessMessage': 'SSH_MSG_CHANNEL_SUCCESS : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
Connected, uploading file: NJ_0615_UPL_130204_101620.expSshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : SendMessage to server 'ChannelDataMessage': 'SSH_MSG_CHANNEL_DATA : #0'.
SshNet.Logging Verbose: 1 : ReceiveMessage from server: 'DisconnectMessage': 'SSH_MSG_DISCONNECT'.
SshNet.Logging Verbose: 1 : Disconnect received: ProtocolError data exceeds negative window tolerance
In debug the error seems inconsistent, it failed 2 out of 3 tries where in release it fails far more often.
Hope this helps.
Rich