Did you search the bugtracker if this issue is known or has been fixed already? If it was not in the bugtracker, did you add it and got a response if this bug was verified and fixed?
The second thing I tried to add the path: /var/www/clients/client2/web6/ftpuser/ to OpenBaseDir
for this website. That hasn't any effect.
Did you add it to the open basedir field in the correct way, separated by : and did you add it in the correct website and not globally?