ls -l /a_dir/.. : permission denied

OK, I am a newbie but ...

from a sub-dir of the root-dir, when I do an "ls -l ..", I get "..: Permission denied".
The same from another directory is OK.
when I "cd /" the do "ls -l /a_dir/.."
/a_dir/..: Permission denied
but "ls -l /" gives no error.
being in / , doing ls -l /.. (as root dir is parent of itself...) is OK.

root-dir has drwxr-xr-x
sub-dir has drwxr-xr-x and was created by root user.

I also get "permission denied with "ls -i .."
I've been googling for a while without being able to find a hint.

Don't know your environment but best ask your administrator. If he is on vacation, post the output of the following commands:

who am i
echo $SHELL
cd /
ls -la |  grep a_dir