Space import authors of last modification to show up as anonymous

(same as https://jira.atlassian.com/browse/CONF-30217, but this was closed).

I run into the same issue when exporting a space, and importing it in another instance of Confluence (both running 5.6.1).

Both have a user by the same name as well, although I cannot imagine that making any difference, since in restrictions, it even names users that do not exist in the second instance.