Hello,
To do concurrent conversions with the conversion plugin based on LibreOffice, you need to use a user profile for each conversion.
Please check the information available on https://www.phpdocx.com/documentation/conversion-plugin/common-problems-and-possible-errors (Concurrent conversions section). Although the information is detailed for Linux servers, the same can be used with Windows and macOS servers. We recommend you use the extraOptions option in your Windows server.
Regards.