Good then!
I think I got the solution. On mac OSX one should just kill the X server (go to activity monitor, look for the xquartz processes and kill them all), then open a shell and give
defaults write org.macosforge.xquartz.X11 enable_iglx -bool true
which would set the -iglx option (whatever it means).
Then restart the ssh -XY session, and run any GL-based application (such as glxgears).
Thank you all for your patience, suggestions and useful help!
Cheers!
Guido