... another user just waiting for the next update that will magically solve everything ...
the problem with smplayer is that either smplayer's development isn't catching up with the new youtube api, or the smplayer version available for linux mint is older than the newest available.
you do, btw, always update your system properly?
there's about half a dozen other solutions to your problem.
i always liked
trizen's youtube viewer with command line & gtk versions.
be sure to look at "Availability", there's a ppa for mint/ubuntu.
aside: i use mpv these days, instead of mplayer. mpv ftw!