This is the version of the server (which is btw. outdated, end of life and no support for it can be given here) and not the version of the sync client.
Thanks a lot! I thought thought the repositories where always up-to-date.
I'll install the client from the owncloud-website.
EDIT: Alas, that didn't work. I get an error, like the one described here: OwnCloud Issue 1907. The proposed solution doesn't work, because the command sudo add-apt-repository ppa:callegar/ppa is not recognised by Raspbian, it seems.
Drifting of topic here, but mentioned it just to be complete.
If you have also installed an outdated version of the sync client (anything below 1.7) you will get this wrong username / password issue. So make sure that you're installing a recent version of the sync client (2.2.3).
I tried to install the most recent client with the instructions on the ownCloud website, but I get an error message and installing fails (see edit in previous post). I give up, I'll just use ftp to sync some important stuff with my webspace. I don't really need all the other options (I just need those files for myself, no sharing is needed, no calendar, contact or music syncing or playing is wanted).