I have had similar problems with Export to CAD using a seed file since I upgraded from 10.3 to 10.6 and from AutoCAD 2013 to 2018 recently. Both upgrades were performed at about the same time (and I have not tried reinstalling the older versions) so I am not sure whether the issue was caused by ArcGIS 10.6 or AutoCAD 2018. Which version of AutoCAD do you have on your computer? ArcGIS 10.6 might be incompatible with AutoCAD 2018 as dwg files appear "empty" and cannot be displayed unless they were first converted to the 2013 version. However, file version does not seem to matter in Export to CAD as the seed file I used was already in the 2013 dwg format. Hopefully there is a solution to this and if not, one can be found soon.
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