New observations (hint to reproduce the problem):

the empty map is shown, when the text size in the browser window is enlarged to values > 100%.
Everything is ok, if text size is smaller than 100%.

I observe the same error as mentioned above.

fails unexpectedly by OpaqueResponseBlocking: "nosniff is true and mimeType is an opaque-blocklisted MIME type or its essence is 'text/plain'

This just means that an HTML json error page was returned instead of the image.

curl 'https://maps.wikimedia.org/img/osm-intl,a,a,a,1200x250@2x.png?lang=de&domain=de.wikipedia.org&title=Vorlage%3AKarte+von+Denkm%C3%A4lern+%28%C3%96sterreich%29&revid=241226507&groups=_cd662694171e2e8dffd9daba590026964ed57f21'

The request returns "image created from bytes must be 2048 pixels or fewer on each side". This is a limit in node_mapnik, used by Maps (Kartotherian) which defaults to 2048. As we limit to 1200, that default is too low and should be 2400.

This is likely due to the usage of width=100%. Combined with some people having retina screens (and thus requesting images twice the size of other users) explaining the user variance.

What's strange is that this was never reported before... I wonder what changed.

Slightly related: T354239

This is likely due to the usage of width=100%

Width is set to 1200px here as workaround to T354239. width=full appears to use 800px wide static image, as Thiemo wrote earlier.

I suppose Kartographer shouldn't allow width values that snapshot service can't handle (both with and without scaling), and snapshot service shouln't try to scale to widths that exceed its width limit?

ok. then set it to 1024. That's the max that will work right now.

I set the max width now to 1024 and it works also when zoom > 100%.
Thanks.

Can somebody please document the limit on all appropriate places.
At least https://www.mediawiki.org/wiki/Help:Extension:Kartographer.