It looks like they’ve really screwed up the account management over at Ekiga.net: I was allowed to create an ID with a password using special characters, but then the Ekiga client failed to log in with it (giving me the error message I titled this entry with) and I couldn’t even log in at the website.
After requesting my password via e-mail (at the login website) as if I had forgotten it, I got an email not with the password but with a URL which, when clicked, gave me a new password. The new one was a measely five characters long; is that how insecure they expect them to be? Well I changed it to yet another one, still 20 characters long but this time containing only numbers and letters, and it seems to have solved the issue: I’m now able to login with the client.