Created attachment 119579 [details] trace of Writer via --backtrace argument. I believe this could have something to do with changes to the gtk filechooser, but I am not certain. Fresh install of Arch Linux along with GNOME 3.18.1 and LibreOffice 5.0.2.2. Problem was not present when running GNOME 3.16. Happens in all applications after selecting "Save As.../a Copy" but before the filechooser dialog appears. Opening files functions. Problem can be worked around by using SAL_USE_VCLPLUGIN=gtk instead.
*** This bug has been marked as a duplicate of bug 94936 ***