rclc_executor_spin_some returns RCL_RET_ERROR when using a custom transport

I am implementing a custom transport on FreeRTOS using the MicroBlaze processor. The transport is straightforward—it continuously polls memory, waiting for new data in the custom read function. I have already verified that the custom read function returns the correct number of bytes.

However, I am encountering the generic error "RCL_RET_ERROR". How can I effectively debug this issue? What does this generic error typically indicate?

Start by tracing rclc_executor_spin_some and the custom read function to find where RCL_RET_ERROR is produced. Reproduce the failure on the FreeRTOS/MicroBlaze transport and inspect the return path and available diagnostics. Done means the failing layer is identified and the issue has a documented correction or debugging path.