In T156804#5477657, @DannyS712 wrote:

Wikis can define the target of help links by creating a page in the mediawiki namespace with the title "lowercase special page name + '-helppage'." - eg a page at MediaWiki:Block-helppage would override the default link

I'm not sure what the issue is but another user reported the same error who uses HitCounter https://www.mediawiki.org/wiki/Extension_talk:HitCounters#Installing_on_1.28