HTTP 429

Too many simultaneous requests

The account, or the key, already has as many requests in flight as its plan allows.

"type": "https://ohlc.io/problems/concurrency-limit"

Why it happens

More parallel requests than the plan's concurrency limit: 2 on Free, 5 on Developer, 20 on Pro and 50 on Commercial and Scale.
Parallel page fan-out across several keys of one account, which share the limit.

Wait for Retry-After seconds, then retry.
Cap your client's parallelism; read X-Concurrency-Limit and X-Concurrency-Remaining.
See limits.