Maybe the same issue as here: unable-to-edit-pages-on-a-fresh-install-local-environment.
We ended up editing this file to get things going: \vendor\silverstripe\assets\templates\SilverStripe\Assets\Flysystem\PublicAssetAdapter_HTAccess.ss
changed regular expression to:
RewriteCond %{REQUEST_URI} !^.*\.(?i:css|js<% loop $AllowedExtensions %>|$Extension<% end_loop %>)$