This indicates you have hit the Box rate limit on creation of new files, Box has a rate limit of 240 files per minute per user, that is 4 files per second.
Here is the list of Box rate limits:
https://developer.box.com/guides/api-calls/permissions-and-errors/rate-limits/
I suggest you take a look at all my responses in this thread where I describe the same phenomena in Google Drive. Just note that the Google Drive specific tuning parameters (--drive-*) will not help on Box.
The best overall advice is to have lots of patience and avoid restarting the job too often.
You may see better results by
- using the default --transfers of 4 to avoid activating additional Box throttling
- increasing --checkers to 16 to speed up the checks when restarting
- mixing the transfer of small and large files by adding --order-by="size,mixed,75"