We are sorry to hear that you have this issue. We need more details in order to resolve the issue, which you have. Please tell us:
1. Which exact version (meaning the assembly version) of GroupDocs.Viewer for .NET are you using?
2. Which project type are you using? Is this WebForms or MVC?
3. Please give us a source code of the GroupDocs.Viewer widget.
4. Please describe, on which stage this error occurs? Which actions lead to the error? If it occurs when you try to display some document, does it depend on the specific document, or document type doesn’t matter?
Thanks and waiting for your reply.