Unknown error when downloading concurrently

[catamounttechnology]

To increase throughput, I start multiple download tasks. Each task downloads ticks for one day. However, if the number of parallel tasks is greater than 3, sometimes a task fails with "Unknown Error".

Potentially the backend is throttling requests from the same endpoint, however "Unknown error" Is not helping at all. A message like "Too many sessions" would be helpful in deciding a strategy (e.g. retry).