I got a mistake with the version that appears on the server's side, and on the webinterface side. It's a fresh 9.0.0 install, which one is upgraded too 9.1.1
Here's the result with the occ command :
root@tempvm:/var/www/owncloud# php occ status -V ownCloud version 9.1.1
here's the result on the page status.php : {"installed":"true","version":"7.0.4.2","versionstring":"7.0.4","edition":""}
Finally, on bottom of the administration page on the web interface, the version is 7.0.4.2 too.
you probably have made a mistake and tried to migrate 7.0.4 from the Debian Repositories to the ownCloud repositories and havn't followed the instructions provided here: