Backend when trying to open the extensions overwiev:

Es ist ein Fehler aufgetreten!
0 Failed opening required '/www/htdocs/w010646d/HHH_Live-Version/components/com_cjforum/helpers/constants.php' (include_path='.:/usr/share/php:..')

Frontend when trying to open the site https://hyperhidrosehilfe.de

0
Failed opening required '/www/htdocs/w010646d/HHH_Live-Version/components/com_cjforum/helpers/constants.php' (include_path='.:/usr/share/php:..')

There was a offer to update to version 6.0.0 direct after 5.1.1 which I have chosen to update.
What can I do now? I am no longer able to use the extension-installer of the backend.