I had this issue today and it was resolved once a specific group (a tree that was appearing in my elevations) was deleted from the drawing file. This file I am working in is a 2018 file, and the tree was imported from an older version of a Vectorworks drawing. When exporting my elevations the file would freeze, but once the tree groups were deleted there was no longer an issue with plotting the drawings to PDF. I am guessing the glitch when exporting happens as a result of my trees being created in a very old version of VW, and then over the past ten years have probably been passed down from version to version before they made it into my current drawing. I have had this same issue before with old symbols created in a 2011 file that were trying to plot in a 2018 file.
In the future: When in doubt, break the group/symbol and just import the line work to avoid this issue if you know you're working with a very old version crossing into a new version. Hope this makes sense!
