Subfolder on meditemple


I have some issues with some deployment on MediaTemple.
So, first the mtr app is installed properly and rails app works fine if
webpath is

But then, by simply changing the web path to:
~$ mtr stop tact
~$ mtr set_option testapp
~$ mtr create_link testapp
~$ mtr start testapp

I have the following error:
Error executing command:
application failed to start, check
/home/12249/containers/rails/testapp/shared/log/mongrel.log for errors

And even more weird, there is no mongrel.log file…

Any quick help is welcome…