What extension are you try to install?
How know you the extension wasn't installed?
Oh! various nir cacheviewr , addfeed, abp etc.
However good news!
I un-installed and then re-installed km totally, then tried ext manager again as an admin. This time it's seems to work! :D
Suggestion: It seems one must first install an ext using the ext manager(in other word, online installer), to d