There is an inconsistency between staging and prod when it comes to empty overlays.
More specifically:
For this snapshot:
/img/osm-intl,5,17.4,8.05,370x310.png?lang=de&domain=de.wikipedia.org&title=Fandou_Béri_(Hamdallaye)&revid=238636115&groups=_2011d7c2cdaee8dd9bcd396dd53e8c58652c8922
Prod returns the snapshot without an overlay because the overlay was empty, in staging it throws this error:
"geojson_datasource: Failed to parse GeoJSON file from in-memory string encountered during parsing of layer 'layer' in Layer"
This is a known issue in latest mapnik:
https://github.com/mapnik/mapnik/issues/4035
In our case we need to try to detect empty overlays and not try to render them