If I press the space bar , the green bar starts growing but nothing moves,
first. see that your Ae software is up to date. it could be just your preview settings. since CC2015 you can customize all your preview shortcuts by many parameters. you probably got cache before playback selected for spacebar. you can reset your preview settings and start fresh.
set it up the way you like. in my setup I want it to behave like the old spacebar.
more about the preview here: Preview without rendering in After Effects
Thank you for the reply, I already tried to reset and to check/uncheck everything in the preview settings but nothing changes. If i press the play button (or tap the space bar) , the play button in preview settings turns to the stop icon but the preview doesn't play, only the caching seems to work but i have to drag the cursor with the mouse to "preview" the clip. Also strange is the fact that if i double tap spacebar, the timeline cursor moves back 1 frame every time i double tap.
Exact Ae version and Os? Try resetting your Ae preferences and clear your cache from media & disk cache