We are sorry to hear that you have this issue. This is the well-known problem, it was fixed in the GroupDocs.Viewer ver 2.0, it is also described in the article “How to Use GroupDocs.Viewer with Streams in ASP.NET MVC or WebForms Projects”.
From the code that you have provided we see that you are using old version of GroupDocs.Viewer for .NET, perhaps version 1.6.
So, in order to fix the issue you need to:
1. Download the latest version of GroupDocs.Viewer for .NET - 2.3.0.
2. Use “Stream” method with parameters “fileName” and “fileExtension”, for example “.Stream(this.fileStream, some_filename, this.fileExtention)”.
3. Remove “DocViewerId” method - it is outdated since version 2.0.
If you will have more questions please feel free to contact us.