OpenSearch Error - 400 Rejected by OpenSearch

Managed to solve this. By default opensearch (and also elasticsearch) plugins log the whole error only if log_level is set to debug. Once I changes the log_level, the errors came with more info about the problem: [warn]: #0 fluent/log.rb:381:warn: dump an error event: error_class=Fluent::Plugin::OpenSearchErrorHandler::OpenSearchError error="400 - Rejected by OpenSearch [error type]: illegal_argument_exception [reason]: 'Validation Failed: 1: this action would add [15] total shards, but this cluster currently has [2997]/[3000] maximum shards open;'"

So to conclude, error "OpenSearchError error="400 - Rejected by OpenSearch" can be for many reasons, and to see more details just change the log level to debug.