I have a rate limit of 600 requests per minute and I was running into it even with a single worker. And 503 is sort of only part of the issue, I suppose it's rather related, but, the bigger issue (could be caused by what causes the 503 though) is that it evidently is rejected by the host when it checks if the file exists and thus it throw that log message about not being able to list/find a file, however, the file is actually there, it just seems that the server refuses to respond (likely due to the aforementioned rate limiting). The 503 also suggests that it is to do with rate limits.