AppSync error: Communication error while executing a request to OpenSearch

Hello,

We have an AppSync API and OpenSearch datasource and resolvers. When we run OpenSearch queries from AppSync resolvers, we get an error in response "Communication error while executing a request to OpenSearch." without any further details. There are no OpenSearch error logs for erred request. We have a DynamoDB datasource that is working without issues. The OpenSearch setup also is working fine with requests from outside AppSync. We are able to access OpenSearch through an EC2 instance launched in the same VPC as OpenSearch. What could be the issue?

Regards,