Limits on uploading contacts to list

I have a large list of contacts (~175,000) to upload to a mailing list. Even when breaking it into small chunks (only 1000 contacts each), I'm running into API errors. It successfully uploads about 4 chunks of 1000, and then I get the following unhelpful error: {"meta":{"error":{}}}

I've tried adding 30 seconds of wait time in between API calls, which slightly helps but doesn't entirely resolve the problem. The documentation states that the API can handle 50 concurrent requests, so I'm not sure why it's failing after about 3 or 4.