After the upgrade i see that the permission of the admin web site is not correct (infact i see the website without css)
I fix it and assign recursive 755 and www-data:www-data as user.
Did you made a backup before, as a recursive chown to this user and group destroys the ispconfig setup. ispconfig sites are never owned by this user or group, in no ispconfig version. Also the folders dont have 755 permissions. With thes echanges you opened up your server for hackers, so I highly recommedn to restore the original permissions.
Regarding your original issue, check the apache error.log