The path is not of a legal form.

In Visual Studio 2005 I got this error when trying to open a form in design mode. This was not a programming error because I was able to properly compile and run my code. After looking at the projects in my solution (luckily, just two simple projects) I have found that one of the projects had a dead reference - a reference with an exclamation mark. I removed that reference and I am now again able to open my form in design mode. I believe a better formulation of the error message would be very helpful in this case.
I post the error stack below, for reference:

The path is not of a legal form.
at System.IO.Path.NormalizePathFast(String path, Boolean fullCheck)
at System.IO.Path.NormalizePath(String path, Boolean fullCheck)
at System.IO.Path.GetFullPathInternal(String path)
at System.Reflection.AssemblyName.GetAssemblyName(String assemblyFile)
Microsoft.VisualStudio.Design.VSTypeResolutionService.AddProjectDependencies(Project project)
Microsoft.VisualStudio.Design.VSTypeResolutionService.AssemblyEntry.get_Assembly()
Microsoft.VisualStudio.Design.VSTypeResolutionService.AssemblyEntry.Search(String fullName, String typeName, Boolean ignoreTypeCase, Assembly& assembly, String description) at
Microsoft.VisualStudio.Design.VSTypeResolutionService.SearchProjectEntries(AssemblyName assemblyName, String typeName, Boolean ignoreTypeCase, Assembly& assembly) at
Microsoft.VisualStudio.Design.VSTypeResolutionService.SearchEntries(AssemblyName assemblyName, String typeName, Boolean ignoreCase, Assembly& assembly, ReferenceType refType) at
Microsoft.VisualStudio.Design.VSTypeResolutionService.GetType(String typeName, Boolean throwOnError, Boolean ignoreCase, ReferenceType refType) at
Microsoft.VisualStudio.Design.Serialization.CodeDom.AggregateTypeResolutionService.GetType(String name, Boolean throwOnError, Boolean ignoreCase) at
Microsoft.VisualStudio.Design.Serialization.CodeDom.AggregateTypeResolutionService.GetType(String name, Boolean throwOnError) at
System.ComponentModel.Design.Serialization.CodeDomSerializerBase.GetType(ITypeResolutionService trs, String name, Dictionary`2 names) at
System.ComponentModel.Design.Serialization.CodeDomSerializerBase.FillStatementTable(IDesignerSerializationManager manager, IDictionary table, Dictionary`2 names, CodeStatementCollection statements, String className) at
System.ComponentModel.Design.Serialization.TypeCodeDomSerializer.Deserialize(IDesignerSerializationManager manager, CodeTypeDeclaration declaration) at
System.ComponentModel.Design.Serialization.CodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager manager) at
Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSerializationManager serializationManager) at
Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.DeferredLoadHandler.Microsoft.VisualStudio.TextManager.Interop.IVsTextBufferDataEvents.OnLoadCompleted(Int32 fReload)