This extention use hook SkinBuildSidebar to inject HTML into the sidebar.
- "As of MediaWiki 1.39, injecting HTML is no longer supported with this hook. If you are looking to inject HTML, such as to put an ad in the sidebar, you must use Manual:Hooks/SkinAfterPortlet or similar."
The extension is therefore partially incompatible with REL1_39+ and must be rewritten to support actual versions of mediawiki.