The Native XML-import plugin does not seem to be stable in 3.1.2 (running in our test environment now).
In my case the import procedure gets interrupted without error warnings at all (error 500). @bozana, I’ve PM’d the XML to you.
UPD: The export procedure for another issue stopped without warnings too (error 500 again).
UPDATE: The errors were caused by some Word files, or custom CSS files associated with the article or some of the editorial stages. Since those files were not essential, I modified the XML file and deleted those specific files tagged within their <submission_file> tags.
I managed to import the files then.