Cannot open SIMP file

Yesterday, I created a sequence diagram using Software Ideas Modeler, but when i try to open it I get this error message and the file does not open:

System.Collections.Generic.KeyNotFoundException: Den givne nøgle var ikke til stede i ordbogen.

ved System.ThrowHelper.ThrowKeyNotFoundException()

ved System.Collections.Generic.Dictionary`2.get_Item(TKey key)

ved SoftwareIdeasModeler.Core.Projecting.Project.BindAbstractItem(ModelElement item)

ved SoftwareIdeasModeler.Core.Projecting.Project.BindAbstractItemsInRepositories(Model model)

ved SoftwareIdeasModeler.Core.Projecting.Project.BindEntities()

ved SoftwareIdeasModeler.ProjectXml.SimXmlReader.Load()

ved SoftwareIdeasModeler.Program.RunMainWindow(Form splash)

When I try to open the backup files, I get the same error.

This is a link to the file I am trying to open:

https://www.dropbox.com/s/4f47nz66s9ofv7a/KlassediagramWebinterface.simp