That's a compatibility issue.
You can use FEA Translator interface as a workaround.
Or you can edit convert_to_cii.bat that you can find in here: ...Everything3D3.1\asso_products\psi\core
You need to set the Template variable so it is compatible with Caesar 2018 and beyond (i.e. set Template=%CoreLoc%\mmtemplate_2018.cii)