This is a known issue. For those of us who used Export>PNG for high rez output, this seems to be broken in the new release of Illustrator CC2018. This seems to only affect people who chose to “Import Previous Settings and Preferences” when upgrading.
Fortunately, Adobe has published a work-around to resolve this issue!
Take a look at this article, to see their answer:
https://helpx.adobe.com/illustrator/kb/troubleshoot-export-issues.html