Since several days, the OSM plugin is showing an empty (gray) map when you zoom in (top z-levels are still working), and is throwing the following error on the console: 

QGeoTileRequestManager: Failed to fetch tile (139022,89584,18) 5 times, giving up. Last error message was: 'Error transferring https://maps.wikimedia.org/osm-intl/18/139022/89584@2x.png - server replied: '

This issue can be easily reproduced by running the Minimal Map (QML) example from the Qt Documentation.

When you open the link in a browser you get this message:

{{Error: 403, Forbidden: Map tiles are restricted to Wikimedia &
affiliated sites only. See
https://wikitech.wikimedia.org/wiki/Maps/External_usage if you believe
your usage supports the Movement.}}

Apperently, Wikimedia now blocks access external access to their WMTS service. It seems that a different tile server is needed.{{ }}