@Volker_E noticed mediawiki.log.js was being removed from his local MediaWiki install. After a lot of head scratching we realized his antivirus Comodo is quarantining the file because it is "Heur.Dual.Extensions" malware. If WIndows is configured to hide extensions, you see a benign-looking mediawiki.log file that would normally open in an editor, but in fact it's a potentially-executable JavaScript file. resources/src is full of other files named something.other.js but this is the only one that "masquerades" as a recognized filetype.
This has come up before, see old Support_desk thread. Also a web search for "mediawiki.log.js" antivirus shows lots of generic ad-filled search results, suggesting that this file is triggering virus warnings on a wide enough scale for spammers to make fake pages for it :-) The fix is easy, rename this one file to mediawiki.logger.js
Surprisingly, his local MediaWiki install worked fine without this file until he tried using a different skin in debug mode, then the early loading of the 'mediawiki' module silently failed with almost no JavaScript loaded. "Silently" as in no errors shown in browser console in regular or RL debug mode. It wasn't until we enabled wgDebugLogFile that we saw
[exception] [43c87615] /load.php?debug=true&lang=de&modules=jquery%2Cmediawiki&only=scripts&skin=blueprint&version=80XVutC9 MWException from line 850 of D:\htdocs\Wikimedia\core\includes\resourceloader\ResourceLoaderFileModule.php: ResourceLoaderFileModule::readScriptFiles: script file not found: "D:\htdocs\Wikimedia\core/resources/src/mediawiki/mediawiki.log.js"
"Almost no JavaScript" as in the browser window has a $ jQuery object but no mw or mediaWiki object, and so all MediaWiki JS enhancements are broken. It feels like a bug in RL that it should be so fundamentally broken without any indication in the browser, but maybe mediawiki.log.js is the file that would report these problems! :-)