Word and Excel document thumbnails

Thumbnails are not related to this plugin, which only affects the viewer pane. (Some plugins also do thumbnails, but not this one.) Opus gets icons/thumbnails for Office documents from the Windows shell, and any shell extensions (usually installed by and part of MS Office) that handle those formats. If they also aren't working in File Explorer either then the problem is not something in Opus. If they only work in Explorer and not Opus, and you're on a current version of Opus, then something is probably blocking the dopus.exe process from accessing your documents.

(Are you sure Word even provides document thumbnails these days? I am not sure that it does.)