Hello Bartosz,
Thursday, March 5, 2020, 10:45:43 AM, you wrote:
Do the files actually exist in those locations? If they don't, and you uploaded them via FTP or something, then just try uploading them again.
Also check the letter case, particularly if you recently upgraded MediaWiki and are using Windows – SpecialWhatLinksHere.php used to be called SpecialWhatlinkshere.php (in all lower case).
OK, looks like it's the letter case thing, thanks a lot!