The summary may not be good, I've tried to guess the cause of the error based on what I knew.

[[:fr:user:Hégésippe Cormier]] merged all his accounts except the one on dz.wiktionary.org. When I tried to attach it manually using [[m:Special:CentralAuth]], I got the error "No unified account for this username." I noticed the field "User name:" had been changed to "Hégésippe_Cormier" (note the underscore), hence my guess about the space character.

Version: unspecified
Severity: normal
URL: http://meta.wikimedia.org/w/index.php?title=Special%3ACentralAuth&target=H%C3%A9g%C3%A9sippe+Cormier