I have WordPress 5.1.1 on the old site. And Version 5.2.3 on the new site.
The old site is still active.
On the import, I did check mark that box “Import Attachments” box.
I’m thinking the error may be on the export side because I can import the xml file on the new site and there are no errors. I get the “All done. Have fun” message. Then I check on posts and there is only the 1 original post from starting the website. But I don’t get any errors on the exporting either. Just select the posts and the date range and hit export and it appears to work…