The Web Gui is broken and I'm quit sure, it already worked in this environment. I upgraded to Ubuntu 16.04 a few weeks ago from 14.04. I can't see my files or e.g. my contacts, see screenshot. Access via Android app or WebDav and syncing with the Windows tool works without any problems.
oC is setting and using a secure CSP by default. If this set different and is showing issues like this the probably some enabled 3rdparty apps are messing around with the default CSP.
I'm having the same issue and found many people blaming apps. However, I'm certain that's not the case, so I'd like to add my two cents to the discussion. I tried disabling all apps, even though I had no 3rd party apps in the first place. So I disabled all apps that I could and it did not help. What helped was uncommenting the following setting in my apache2.conf:
Header set Content-Security-Policy "default-src 'self';"
I configured it manually and globally as it is recommended to secure a web server. When i have some time i will try to exclude only oC by using e.g. virtual hosts.