Sim 9.30-project does not open

Today I have downloaded SIM 9.30 and worked a few hours with it, without problems, but could not reopen the sim-file afterwards. I am using the 64 bit, portable version, running on Windows 10. Working with older SIM versions has never caused a problem like this.

Opening the file results in the following error message:

System.NullReferenceException: De objectverwijzing is niet op een exemplaar van een object ingesteld.

bij SoftwareIdeasModeler.Core.DiagramItems.Concrete.ConcreteRelation.UpdatePoints()

bij SoftwareIdeasModeler.Core.DiagramItems.Concrete.ConcreteRelation.CalculateLayout(DiagramDrawer drawer, String relationText, Boolean renderName)

bij SoftwareIdeasModeler.Core.DiagramItems.UmlObjects.ConcreteUmlRelation.CalculateLayout(DiagramDrawer drawer, String relationText, Boolean rendersName)

bij SoftwareIdeasModeler.Core.DiagramItems.UmlObjects.ConcreteUmlRelation.RefreshLayout()

bij SoftwareIdeasModeler.Core.Projecting.Project.RefreshLayout()

bij SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

bij SoftwareIdeasModeler.ProjectHelper.LoadProject(String filename, Boolean showReadOnlyWarning, Boolean checkFileVersion, Boolean processCustomDiagrams)

bij SoftwareIdeasModeler.ProjectHelper.OpenProject(MainForm mainForm, String filename)