Sorry to hear about the trouble. As you mentioned, it was working in the previous version, but now reverting to the previous version doesn't help. You may try resetting Illustrator Preferences (https://helpx.adobe.com/illustrator/using/setting-preferences.html) and see if that helps. If it doesn't, then I will suggest you to contact the DXF conversion Software company or try installing an older version of that software as well. I hope it helps! Disclaimer: Please note that resetting preferences will remove all the custom settings, and Illustrator will launch with default settings. You can also take a backup of the folders in case you want to. Location is mentioned in the article.