Thank you for contacting support. Please provide us your problematic Visio diagrams in this forum thread. We will investigate and reply you accordingly. You can make a zip of the problematic diagrams and attach to your reply.
Thank you for posting sample Visio diagrams. We have tested them against the latest build of Aspose.Diagram for .NET 5.6.0 and did not notice any error message. We have attached the resultant documents (zip file) for your reference. Please download and use the latest version of Aspose.Diagram for .NET 5.6.0.
We hope, this helps. Please let us know in case of any ambiguity or questions.