The Wikipedia iOS app pulls Picture of the Day information from an API call that looks like this: https://commons.wikimedia.org/w/api.php?action=query&format=json&generator=images&iiextmetadatafilter=License|LicenseUrl|LicenseShortName|ImageDescription|Artist&iiextmetadatalanguage=en&iiextmetadatamultilang=1&iiprop=url|extmetadata|dimensions&iiurlwidth=640&prop=imageinfo&rawcontinue&titles=Template:Potd%2F2020-12-14_(en) Adjusting the date and language abbreviation at the end of that URL allows you to change the day and language.
For 2020-12-14, en and de provide an empty result, but not an error in the call. This led to crashes in the iOS app. (There is a separate ticket to improve the iOS app so that this situation will not cause a crash in the future.) There are no images in the next few weeks that provide issues with English imageinfo besides 2020-12-14, but it would be good to figure out what broke in this situation.
(Notably, there was some old markup on the 2020-12-14 image that was cleaned up on 2020-12-14 to see if that fixed the problem. Even when the cache was invalidated, the blank API response remained.)