w:Template:Usercheck-short and probably other templates use {{urlencode:}} to encode a username, e.g. "Example User" becomes "Example+User" when generating a link to the EC (e.g. https://xtools.wmflabs.org/ec/en.wikipedia.org/Example%2BUser. However, following the link does not work because EC claims the user does not exist.
Topic on Talk:XTools
That is because https://meta.wikimedia.org/wiki/Special:CentralAuth/Example+User doesn't exist - the problem is on the wiki encoding side. It should be %20, not %2B... :(
Yeah, I figured that out now. I added PATH to the urlencode: and it now encodes as %20 :-)