The Wikimedia Foundation’s Product and Technology team has notified the OpenStreetMap Foundation system administrators that access to Wikimedia APIs for the OSM wiki may be restricted due to inefficient use of Wikimedia Commons images. To avoid a potential block, the team has requested that image requests be limited to standard thumbnail sizes. As a result, minor adjustments to thumbnail dimensions may appear across the wiki in the coming days, and any resulting layout issues are being monitored.

I think this might happen to you if your MediaWiki wiki is extremely large and doing the same thing to the OSM wiki.