If Divi library export is not working or downloading a kb export file then opening the dashboard in a new incognito chrome browser window worked for me.
Also if you have a large number of layouts then repeat this process by closing and opening the incognito tab, and logging into the dashboard, and then exporting another set of marked layouts the same way.
If you have installed w3totalcache plugin, you can also try disabling this plugin and re-enabling it to empty the cache. It did helped me exporting the divi layout files without getting that 0 kb file.