I’m sorry for the late reply but we were unable to reproduce the error that you were getting. Now we finally know what is causing it and will include a fix in the next release. The error is triggered if some folder on a path to the imported .obj file is using national or some other character outside of the standard ASCII set.
As a temporary workaround you can move the matterpack to a folder that doesn’t use any such characters and doesn’t have any parent folder with such characters.