I had a very similar problem recently. It turned out to be a newly installed plug-in. Unfortunately, I don’t remember which one except that it had something to do with showing hidden objects. You can disable or uninstall plugins one at a time until things start working correctly, at least, that’s what I did.
I see you have the plugin manager installed. Try disabling all except the ones installed as part of the SU install which have the preface SU_. If that works ok enable 1/2 of 1/2 of the ones disabled, if ok that means the other half has culprit. Enable another 1/2 etc This binary search lets you quickly go thru all the ones you have. Of course this only works if 1 only is problem.
Ok I found the problem was about MSPhysics which is not compatible with 2015 ( see GitHub - AntonSynytsia/MSPhysics: A NewtonDynamics wrapper for SketchUp.). IT IS NOT A POOR WRITEN SCRIPT, I just got it in the Ruby Depot without reading carefully AntonSynytsia’s page.
Thanks a lot everybody.