thanks for the question and welcome to the Rebex forum.
From the description provided, I can only guess that an exception has been thrown from the Azure BlobClient (Azure stream). We had encountered a misbehaving stream in previous versions of the Azure SDK (unsupported SetLength method, problematic Seek method).
Server log should reveal where the problem lies.
To create a server log, use code similar to code on the following line.
mySftpServer.LogWriter = new Rebex.FileLogWriter(@"c:\temp\log.txt", Rebex.LogLevel.Debug);
Alternatively, you can log file operations in your file system provider. Your provider should have a constructor that accepts an instance of the Rebex FileSystemProviderSettings.
public MyAzureProvider(FileSystemProviderSettings fileSystemSettings = null) : base(fileSystemSettings)
Use logger in the provider.
var settings = new FileSystemProviderSettings()
LogWriter = new Rebex.FileLogWriter(@"c:\temp\log.txt", Rebex.LogLevel.Debug)
var myAzureProvider = new MyAzureProvider(settings);