Ubuntu上的bundler权限错误（ruby使用apt，NOT rvm）

We used to have a redmine setup on an Ubuntu 12 based server but our hoster upgraded the servers and we now have to move everything to the new machine. It now runs under Ubuntu 14LTS. I've installed the LAMP package with apt as well as the redmine one and (I assume) all dependencies required (including Passenger). I did the modifications on the Apache2 conf files and I'm now able to access the redmine site... only to get the error page showing the following:We used to have a redmine setup on an Ubuntu 12