AWS DMS: unable to select the RDS for engine Oracle

In DMS, when creating either a Data Provider or Endpoint (source), my Oracle RDS (19c, Standard Two, CDB) is not visible as a choice. A different RDS (Postgres) from same VPC, same subnets (private), same security groups, same AZ (eu-central-1), is visible and accessible. If I enter the connection manually, then DMS still fails to connect during Schema Conversion. What else I tried without success: explicit IAM policy/role for DMS & DB's ARN; editing the security group (for all IPv4 TCP and my custom port).