Ah! I wasn't aware the installer embeds paths into the shared objects. I moved the files around a bit after the install so now they've got invalid paths in them.

Thanks for the info!

[Paths]
Plugins=plugins

or you could binary edit libQt5Core.so.5, it has a setting created by Qt's installer that points to 5.6.1's prefix directory, to see it type strings libQt5Core.so.5 | grep prfxpath
(Qt adds plugins to that path to get 5.6.1's plugins)

or you could do a call to QCoreApplication::addLibraryPath("plugins"); just before QApplication a(argc, argv); in your main.cpp

and as a final option, which might be the easiest, instead of having a directory structure with a plugins directory containing the platforms subdirectory etc.. move all those subdirectories up one level, so that there is a platforms directory visible from your application's .exe file.
(That would be equivalent of creating a qt.conf file with this contents:

[Paths]
Plugins=.

Hope this helps!

ldd doesn't show anything missing. QTDIR is set to the qt 5.6.1 dir which contains lib, include, bin, plugins, etc.

The error:

$ LD_LIBRARY_PATH=. ./application
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Reinstalling the application may fix this problem.
Aborted (core dumped)

ldd output:

$ LD_LIBRARY_PATH=. ldd ./application 
	linux-vdso.so.1 =>  (0x00007ffd6068b000)
	libApplication.so.1 => ./libApplication.so.1 (0x00007ffa421b9000)
	libhunspell.so.1 => ./libhunspell.so.1 (0x00007ffa3e54e000)
	libprotobuf.so.8 => ./libprotobuf.so.8 (0x00007ffa3e104000)
	libopencv_core.so.3.0 => ./libopencv_core.so.3.0 (0x00007ffa3dbfd000)
	libQt5Widgets.so.5 => <QTDIR>/lib/libQt5Widgets.so.5 (0x00007ffa3d386000)
	libQt5Gui.so.5 => <QTDIR>/lib/libQt5Gui.so.5 (0x00007ffa3cb8d000)
	libQt5Xml.so.5 => <QTDIR>/lib/libQt5Xml.so.5 (0x00007ffa3c952000)
	libQt5Core.so.5 => <QTDIR>/lib/libQt5Core.so.5 (0x00007ffa3c23e000)
	libGL.so.1 => /lib64/libGL.so.1 (0x00007ffa3bf8d000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007ffa3bd71000)
	libstdc++.so.6 => /lib64/libstdc++.so.6 (0x00007ffa3ba69000)
	libm.so.6 => /lib64/libm.so.6 (0x00007ffa3b766000)
	libgcc_s.so.1 => /lib64/libgcc_s.so.1 (0x00007ffa3b550000)
	libc.so.6 => /lib64/libc.so.6 (0x00007ffa3b18e000)
	libuuid.so.1 => /lib64/libuuid.so.1 (0x00007ffa3af88000)
	librt.so.1 => /lib64/librt.so.1 (0x00007ffa3ad80000)
	libQt5XmlPatterns.so.5 => <QTDIR>/lib/libQt5XmlPatterns.so.5 (0x00007ffa3a7ab000)
	libQt5Network.so.5 => <QTDIR>/lib/libQt5Network.so.5 (0x00007ffa3a44a000)
	libQt5Sql.so.5 => <QTDIR>/lib/libQt5Sql.so.5 (0x00007ffa3a206000)
	libssl.so.1.0.0 => ./libssl.so.1.0.0 (0x00007ffa39f94000)
	libcrypto.so.1.0.0 => ./libcrypto.so.1.0.0 (0x00007ffa36a4f000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007ffa3684b000)
	libz.so.1 => /lib64/libz.so.1 (0x00007ffa36635000)
	libgobject-2.0.so.0 => /lib64/libgobject-2.0.so.0 (0x00007ffa363e4000)
	libgthread-2.0.so.0 => /lib64/libgthread-2.0.so.0 (0x00007ffa361e2000)
	libglib-2.0.so.0 => /lib64/libglib-2.0.so.0 (0x00007ffa35eab000)
	libXext.so.6 => /lib64/libXext.so.6 (0x00007ffa35c98000)
	libX11.so.6 => /lib64/libX11.so.6 (0x00007ffa3595a000)
	libicui18n.so.56 => ./libicui18n.so.56 (0x00007ffa354bf000)
	libicuuc.so.56 => ./libicuuc.so.56 (0x00007ffa35107000)
	libicudata.so.56 => ./libicudata.so.56 (0x00007ffa33724000)
	/lib64/ld-linux-x86-64.so.2 (0x00007ffa4354d000)
	libexpat.so.1 => /lib64/libexpat.so.1 (0x00007ffa334f9000)
	libglapi.so.0 => /lib64/libglapi.so.0 (0x00007ffa332cb000)
	libselinux.so.1 => /lib64/libselinux.so.1 (0x00007ffa330a6000)
	libXdamage.so.1 => /lib64/libXdamage.so.1 (0x00007ffa32ea2000)
	libXfixes.so.3 => /lib64/libXfixes.so.3 (0x00007ffa32c9c000)
	libX11-xcb.so.1 => /lib64/libX11-xcb.so.1 (0x00007ffa32a9a000)
	libxcb-glx.so.0 => /lib64/libxcb-glx.so.0 (0x00007ffa3287f000)
	libxcb-dri2.so.0 => /lib64/libxcb-dri2.so.0 (0x00007ffa3267a000)
	libxcb-dri3.so.0 => /lib64/libxcb-dri3.so.0 (0x00007ffa32477000)
	libxcb-present.so.0 => /lib64/libxcb-present.so.0 (0x00007ffa32273000)
	libxcb-randr.so.0 => /lib64/libxcb-randr.so.0 (0x00007ffa32065000)
	libxcb-xfixes.so.0 => /lib64/libxcb-xfixes.so.0 (0x00007ffa31e5d000)
	libxcb-render.so.0 => /lib64/libxcb-render.so.0 (0x00007ffa31c52000)
	libxcb-shape.so.0 => /lib64/libxcb-shape.so.0 (0x00007ffa31a4e000)
	libxcb-sync.so.1 => /lib64/libxcb-sync.so.1 (0x00007ffa31847000)
	libxcb.so.1 => /lib64/libxcb.so.1 (0x00007ffa31624000)
	libxshmfence.so.1 => /lib64/libxshmfence.so.1 (0x00007ffa31421000)
	libXxf86vm.so.1 => /lib64/libXxf86vm.so.1 (0x00007ffa3121b000)
	libdrm.so.2 => /lib64/libdrm.so.2 (0x00007ffa3100d000)
	libffi.so.6 => /lib64/libffi.so.6 (0x00007ffa30e04000)
	libpcre.so.1 => /lib64/libpcre.so.1 (0x00007ffa30ba3000)
	liblzma.so.5 => /lib64/liblzma.so.5 (0x00007ffa3097d000)
	libXau.so.6 => /lib64/libXau.so.6 (0x00007ffa30779000)

If I set QT_QPA_PLATFORM_PLUGIN_PATH it works.

QT_QPA_PLATFORM_PLUGIN_PATH=$QTDIR/plugins LD_LIBRARY_PATH=. ./application
-- application runs without errors --

This started happening when we switched from qt 5.2.1 to 5.6.1-1. We would build qt 5.2.1 ourselves from source (so we could link our own build of SSL libs). On 5.6.1 we're using the qt installer to get the libs instead of building them ourselves because we no longer need to use custom built SSL libs.

We have a plugins directory relative to the application binary. So I thought maybe that was confusing Qt's plugin path search, so I've tried linking the content of QTDIR/plugins into the application plugins directory:

ln -s $QTDIR/plugins/* ./plugins/

But that didn't work. The only thing I've found, so far, that works is setting QT_QPA_PLATFORM_PLUGIN_PATH. We can do that, but it seems like there's something wrong here and I'd like to figure out what it is.

Any ideas?