Owncloud 4 was just released. The update was straight forward, although unfortunately each login redirected me again on the login page with no error. Furthermore, the log was rather empty.
After a while, I stumbled over this line within the log:
302 means that somewhere a redirect occurred hinting me to the ssl-proxy running on our machine.
Short usage of google pointed me to http://bugs.owncloud.org/thebuggenie/owncloud/issues/oc-762, describing the exact problem I had.
1. Adapting the base.php within owncloudinstallation/lib/base.php somewhere near 372 and replacing the line as described in the bug report:
2. Adapting the virtualhost of the owncloud location:
3. Adapting the virtualhost of the ssl-proxy:
That’s it, Owncloud 4 now runs behind our ssl-proxy.