@julienG great question! This is likely because there's a code block timeout of 15 minutes (900000ms). So that specific block within your workflow must have timed out. Just in case it'd be useful, you could check out the Retry feature in the block's Settings to make sure that a block is retried on some interval if it fails.
I know the Workflows team is also planning to publish more documentation on the limits and timeouts that exist, as this would definitely ideally be included in our Workflows docs.