Thread:Project:Support desk/fatal error message: SemanticMediaWiki/reply (3)

Appanrently, this is not a SMW bug.

addHTMLclass::hAddHtmlTag is part of Extension:AddHTML, which is unmaintained (and apparently incompatible with current version of MediaWiki). That causes the warning, which probably causes some other errors in the code which ends in SMW to not load properly all the required classes.

Try disabling the AddHTML extension.