Identified at https://www.wikidata.org/wiki/Wikidata:WikiProject_sum_of_all_paintings/Property_statistics/Sandbox − although the crash was silent, just resulting in a 502. Had to reproduce manually directly in property_statistics.py.
When grouping_link is used, integraality looks up the English label of the item (which is debatable, and under re-consideration at T237276: Re-evaluate the grouping-link behaviour ).
However, if the item has no English label, then this fails with:
Traceback (most recent call last):
File "./integraality/property_statistics.py", line 681, in <module>
main()
File "./integraality/property_statistics.py", line 677, in main
print(stats.retrieve_and_process_data())
File "./integraality/property_statistics.py", line 619, in retrieve_and_process_data
text += self.make_stats_for_one_grouping(grouping, item_count, higher_grouping)
File "./integraality/property_statistics.py", line 546, in make_stats_for_one_grouping
label = group_item.labels["en"]
File "/usr/local/lib/python3.7/site-packages/pywikibot/page/_collections.py", line 46, in __getitem__
return self._data[key]
KeyError: 'en'