Screenshot of problem.
On mediawiki.org I just got the following notification:
|[[User:[Nobody]|[Nobody]]] thanked you for your edit|
|on MediaWiki talk:Gadget-UTCLiveClock.js.|
|1 day ago||View edit|
Edit in question:
Whatever the case, the main issue is that [Nobody] doesn't parse when used inside [[User:]] in wikitext (at least not in the parser Echo uses for this). So either it should be replaced with a placeholder, or perhaps the notification should be hidden entirely if it doesn't have sufficient data.
Either way, it should not be communicating a transaction with user=[Nobody] because that is an invalid username and also invalid page title for user pages in general. Presumably the latter is on purpose, (so that it can't point to an existing user, though we do have reserved usernames for certain purposes), but then it shouldn't try to parse it as such inside the notification.