Uploading a big csv file with 500,000 records fails after 25% and is disconnected.

How long is the upload taking you? Is your internet on the slow side? It doesn't seem like the file size would be any kind of issue but that really depends on the data within. How many MB is the file?

From what I've seen it can be a combination of many things. If the app is under heavy use, the file is huge, or your internet connection is slow you can end up timing out. Any combination of those really.