Parallel file upload fails

That error can occur in some cases, in which case the response status code will be 503. If the response status code is non-200, you should consider the upload failed.

This error generally indicates that there was simultaneous activity in the account preventing your app from making the state-modifying call (e.g., adding or removing files) it is attempting. The simultaneous activity could be coming from your app itself, or elsewhere, e.g., from a user's desktop client.

You can't queue these on our side, so the best practice is to reduce the number of simultaneous requests and to just retry a request, respecting the Retry-After header if given, or after a small delay possibly with an exponential back-off, when receiving this error.