I get this everytime (I’m using the latest MG dev) :
H:/________MonoGame/SanMiguelTest/sanMiguel.obj: error: Importer ‘OpenAssetImporter’ had unexpected failure!
Assimp.AssimpException: Error importing file: The string “” cannot be converted into a value.
à Assimp.AssimpContext.ImportFile(String file, PostProcessSteps postProcessFlags)
à Microsoft.Xna.Framework.Content.Pipeline.OpenAssetImporter.Import(String filename, ContentImporterContext context)
à Microsoft.Xna.Framework.Content.Pipeline.ContentImporter`1.Microsoft.Xna.Framework.Content.Pipeline.IContentImporter.Import(String filename, ContentImporterContext context)
à MonoGame.Framework.Content.Pipeline.Builder.PipelineManager.ProcessContent(PipelineBuildEvent pipelineEvent)
Build 0 succeeded, 1 failed.
When I edit the obj file with notepad++, there is no match found for “” in it !
Just a wild guess here but it could be that the importer does not trim starting and trailing spaces from asset names/linked names and there is a space at the front of something?
Not even a single " is present in the file. Nor in the mtl.
Maybe it is just a wrong message, and should be a sort of overflow (the model’s file is 1.8GB)
Fbx converter has just finished… Pipeline building…
Hmm best wait for someone with more knowledge relating to this then…
But if I recall correctly most of the linked asset addressing is at the top of a file so double check the addressed files if any… or is this error not relating to the loaded data?
Ah I best leave it here… not been a good week for me… sorry.
it is what I meant by Assets… but I think you got my meaning 
no need of words 
in 2014.