'System.NullReferenceException' occurred in WeOn (General questions)

When I execute the following code from a console application, it works fine (connects to server, transfers, etc.)
When I execute it on a background thread from a Windows Form I get the subject exception on the line:
new WeOnlyDo.Client.SFTP

I verified that the license key string is retrieved from the (encrypted) config file.

Here is a snippet:

Configuration appConfig = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

ConnectionStringsSection connSection = appConfig.ConnectionStrings;

uploadServer.sftpClient = new WeOnlyDo.Client.SFTP();

// for use of WeOnlyDo DLL
uploadServer.sftpClient.LicenseKey = connSection.ConnectionStrings[ LicenseKey ].ConnectionString;

uploadServer.sftpClient.Hostname = connSection.ConnectionStrings[ ISSServer ].ConnectionString;

uploadServer.sftpClient.Login = connSection.ConnectionStrings[ TestUser ].ConnectionString;

uploadServer.sftpClient.Password = connSection.ConnectionStrings[ TestPassword ].ConnectionString;

uploadServer.sftpClient.Blocking = true; // not asynchronous
uploadServer.sftpClient.Timeout = 60; // autodisconnect on inactivity in seconds, 0 means no timeout.

uploadServer.Connect(remotePath, uploadWorker);