See https://www.mediawiki.org/w/index.php?title=Special:Log&type=usermerge
16:58, 1 August 2017 DerHexer (talk | contribs | block) merged the account "Yuvipanda" (349102) into "User:YuviPanda|YuviPanda" (2484972)
Either we remove the wikisyntax from 'usermerge-success-log' message or we fix UserMerge to parse that correctly.