I noticed the bug the first time at pl.wikipedia on 2018-10-24 at 06:52 CEST.
Steps to reproduce (tested with de.WP and en.WP):
- Use Special:Export to export an article with the full history (uncheck "Include only the current revision, not the full history")
The XML is broken (check with a XML validator or a browser):
- The element type "page" must be terminated by the matching end-tag "</page>".
- "Error: mismatched tag . Expected: </page>
Exporting only the current revision seems to work (valid XML).