I've doubled checked the official release of DOCXImport_v0.2.8.zip from its GitHub release page (
https://github.com/dougmassay/docxim...eleases/latest) against the DOCXImport_v0.2.8 from the plugins thread here at Mobileread (
https://www.mobileread.com/forums/sh...d.php?t=273966), and the zip archives are identical. The question becomes what you are doing in order to get a malformed/misnamed zip archive installed as a plugin with Sigil's official 'Add Plugin' feature, which will resist installing plugins that don't meet strict naming and structural requirements.
Are you manually modifying the zip you're mistakenly getting from github in order to get it to install, or are you unzipping and manually moving its contents into the plugins folder of Sigil's preferences location?