Addition:
As the board in question is located in Finland, the use of scandinavian chars are quite common (?, ?, ?). On top of that we have a lot of users with usernames containing: *, _, -, <, >, ^, +, [, ], #… etc.
For phpBB this is not a problem, as it accepts almost anything in the usernames by default. But WP seems to dislike almost anything except alphanumerical’s and spaces, and the same seems to go for the CommunityAPI SSO -plugin.