Guillermo, I have seen this error, and thought it was fixed in the 1.55 version of the AutoCAD plugin, but clearly not. I will need to investigate further over the next day or so. In the meantime, I recommend rolling back to the previous version of the plugin.I got problems with the latest plugin version 1.55, it seems that it does not save materials, and it shows this message when I open the plugin tab, also it never stops showing the message loading materials. At the end it does not load the saved materials.Please check.
Paul