I have seen it in AC10 and AC11. I am pretty sure it is somehow related to the input sequence of file name and the file/directory structure when exporting. It is something like if ArchiCAD is already looking at a directory and you change the name or format (Artl, psd, etc.) of the export (as opposed to changing the name and then browsing to look up the destination directory) you get that message. It happened to me twice a few minutes ago, and things somehow work if I rebuild the 3D window, 'Save As', change the export file name if needed, browse to look up the directory.