This issue has been reported to Dwarden and I've been informed its logged internally - Wanted to log it here so its tracked publicly.
The Headless Client RPT log file will produce the following error (at a rapid rate thus increasing the log file significantlly if the -nologs param isn't used), the following error:
Invalid post effect handle.
This has been occurring for many verions (possibly from the start of HC being able to work with A3).
I've attached 'steamonline.zip' which contains my DS + HC RPT files, the server and network configuration files, and startup scripts for my DS + HC. {F21469} {F21470}