If you have access to the filesystem the solution is simple:

chmod +x / usr

With chmod -x you disallowed listing of the directory contents.
With chmod +x you allow listing them.
chmod takes each parameter as argument and tried to remove the listing right from "/" (the root directory) "usr" "share" "sheep" and "startup" files/directories. Only the first two where accessible from the root path so only these two are affected.

If you don't know how to access the filesystem of the N9 take a look here: https://wiki.maemo.org/N9/N950_Rescue_CD

Additional info: If you chmod -x on a file you forbid execution of it, so a program can not be started anymore. With chmod +x you allow it.