Seems as if you started the joomla installation without suexec and then switched to suexec without closing the webbrower in the meantime. This causes that joomla has written a session that belongs to the apache user first and then is not able to change it as web user.
There are several solutions:
a) Close all browser windows.
rm -f /var/www/clients/client2/web2/tmp/sess_*
to remove all sessions.
Had to tail the error.log 'cause it was causing memory crashing with nano and vi!!
For logfiles, you should use always tail and not an editor to view it.