[HUGE PROBLEM] I can't install addons

Hi all,

I have a problem which started driving me mad. I am preparing Blender to cooperate with it smoothly - addons, scripts, keyconfigs, to work as fast as possible. The problem is, in some unidentified place in time, addons are not able to install. I don’t see them in the list in all categories (official, community, testing), even though they were visible before. I have working internet connection, latest stable build 2.74, and of course - I did re-install in many different localizations including erasing all user data and registry keys. Nothing works, nothing helps, I just can’t install them. Even if the program says (in console) it installed an addon or script, it’s not there, I can’t find it nor I can configure it.

Did anyone have such problem? I need help, would be grateful for some support or ideas to get it fixed, I tried almost everything.

(WINDOWS 8.1 x64)

Do you use the installer or the ZIP file Blender download for Windows?