Added Thumbor tag for additional insight.

Can't find anything wrong (or anything relevant having changed recently) in UploadWizard's Flickr workflow.
The upload seems to be submitted just fine, with (example) params like these:
{ action: 'upload', stash: 1, url: 'https://live.staticflickr.com/1877/43706077435_d8cea9317a_o.jpg', filename: 'CHERNOBYL 30 YEARS AFTER — Public Domain CC0 - 43706077435.jpg' }

Upload API response seems equally normal. Example response.upload.imageinfo.url = https://commons.wikimedia.org/wiki/Special:UploadStash/file/16ikzdeqdbe4.19rb6v.1974855. (Note: 500, with "Cannot serve a file larger than 1048576 bytes.", but that's ok AFAIK)

A follow-up ?action=query&prop=stashimageinfo also responds as I'd expect, with a (example) response.query.stashimageinfo[0].thumburl = https://commons.wikimedia.org/wiki/Special:UploadStash/thumb/16ikzdeqdbe4.19rb6v.1974855./120px-16ikzdeqdbe4.19rb6v.1974855..jpeg

Unfortunately, there is no thumbnail at that location, but this error:

Not Found
Fetching thumbnail failed: Array ( [0] => Array ( [0] => http-bad-status [1] => 404 [2] => Not Found ) ) URL = https://upload.wikimedia.org/wikipedia/commons/thumb/temp/8/88/20190405101438%21URL18a98d9cee64.urlupload_./120px-20190405101438%21URL18a98d9cee64.urlupload_..jpeg

From what I can tell, the upload appears to have gone though fine (or if there was an error in there, it wasn't caught), but there is an issue while thumbnailing the file.
FWIW, this seems to work just fine on beta (though I'm not sure if thumbnailing is set up differently there)

It's sort of fixed right now, as we have the fix running on 1 out of 3 thumbnailing servers and Upload Wizard keeps retrying on failure. Sooner or later it hits the server that has the fix and the thumbnail shows up. Should get deployed to all servers tomorrow.