Everything seems ok in smplayer except that. Youtube also works in internet browsers.

Changing smtube settings doesn't help

YouTube changed the way it outputs HTML, so SMPlayer can't retrieve the videos URL anymore (it's the same problem for VLC, see my answer for this here)
Until an updated SMPLayer is released, it appears the only way is compiling the newest version yourself, since, unlike in VLC, the video retrieval method is hardcoded in C++.
The problem is fixed in smplayer 0.8.1 and smtube 1.2. You can get packages from this ppa: https://launchpad.net/~rvm/+archive/smplayer