Please check the minimum system requirements System requirements for Photoshop Lightroom for Mac and Windows OS for Lightroom 6/2015 and compare it with the mac you have also, turn off the GPU from the Lightroom preferences and check if that helps.
Open Lightroom > Edit Menu > Preferences > Performance tab > Uncheck GPU > Restart.
Thanks for your answer.
Everything was good to work easily on Lr until now, with my mac. I didn't update Lr thoses last days, so no reason for my Mac to be now not compatible with this version of Lr.
I've tryied to turn off the GPU, but, still have my problem.
I really don't know what i have to do..