I am using Libreoffice 5.2 (but I don't more than that). It was working yesterday. I have installed MacOS Sierra. Now when I start Libreoffice, I can get to the basic screen that asks me what I'd like to create, and lists recently used documents, but if click any of the documents, or attempt to see what version of Libreoffice I am using using the menus, it freezes.
I just downloaded and installed LibreOffice_5.2.1MacOS_x86-64.dmg and I get the same error.
I just noticed this comment in a bug report that says the same thing [https://bugs.documentfoundation.org/show_bug.cgi?id=100995#c7] My machine too is a iMac 5K (Late 2015).
*** This bug has been marked as a duplicate of bug 100994 ***