Upon investigating a suspected bug for a user today, I found it was caused by the user's document in BookMacster having two folders with exactly the same name in the Reading List.

Short Version.  If you get this error when exporting to Safari, see if you have two folders of the same name in your Root or Reading List or Other Bookmarks.  If so, change the name(s) so that all such folders have different names.

Details

Because Safari does not allow folders in Reading List, BookMacster remaps these into the Root level.  Well, it turns out that macOS will not allow us to put two folders of exactly the same name at root level; hence the error 772030 Cannot create a new item (which refers to the second such folder) when we try to export such an arrangement from BookMacster, Markster, Smarky or Synkmark.

This behavior is due to some unexpected behavior in Safari, which I think is a bug, but maybe Apple thinks it is a feature.  To reproduce this behavior, create such duplicate-named folders in Safari's Edit Bookmarks window.  It will look OK at first, but the next time you relaunch Safari, one duplicate-named folder will have disappeared, and its contents will have been combined into the other folder.  Or you can watch me do it.

This behavior is reproducible in macOS 10.14.5, Safari Version 12.1.2 (14607.3.1), but maybe in other recent versions too.