Description:
I'm using Debian GNU/Linux 9.9 (stretch) with development version of LibreOffice 6.2.6.1.

When I run libreoffice6.2 under KDE session with default environment (no SAL_USE_VCLPLUGIN specified), it starts as expected and I can see main window with recent documents on it.

If I specify SAL_USE_VCLPLUGIN=qt5 libreoffice6.2 as start command it hangs on splash screen.

I've tried to run libreoffice6.2 using gdb to find out backtrace of hanged function and I've got following results:
$ SAL_USE_VCLPLUGIN=qt5 gdb --args /opt/libreoffice6.2/program/soffice.bin

... GDB output skipped ...

Thread 1 "soffice.bin" received signal SIGINT, Interrupt.
pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
185     ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S: Нет такого файла или каталога.
(gdb) info threads
  Id   Target Id         Frame 
* 1    Thread 0x7ffff7f01fc0 (LWP 7006) "soffice.bin" pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
  2    Thread 0x7fffdace8700 (LWP 7012) "QXcbEventReader" 0x00007ffff34d88bd in poll () at ../sysdeps/unix/syscall-template.S:84
  3    Thread 0x7fffce6bc700 (LWP 7013) "QDBusConnection" 0x00007ffff34d88bd in poll () at ../sysdeps/unix/syscall-template.S:84
  5    Thread 0x7fffcd13e700 (LWP 7015) "PipeIPC" 0x00007ffff34e29ad in accept () at ../sysdeps/unix/syscall-template.S:84
(gdb) bt
#0  pthread_cond_wait@@GLIBC_2.3.2 () at ../sysdeps/unix/sysv/linux/x86_64/pthread_cond_wait.S:185
#1  0x00007fffec27750c in std::condition_variable::wait(std::unique_lock<std::mutex>&) () from /usr/lib/x86_64-linux-gnu/libstdc++.so.6
#2  0x00007fffe2a8cd8b in Qt5Instance::RunInMainThread(std::function<void ()>) () from /opt/libreoffice6.2/program/libvclplug_qt5lo.so
#3  0x00007fffe2a8161c in Qt5Frame::Show(bool, bool) () from /opt/libreoffice6.2/program/libvclplug_qt5lo.so
#4  0x00007ffff64deb9f in vcl::Window::Show(bool, ShowFlags) () from /opt/libreoffice6.2/program/libmergedlo.so
#5  0x00007ffff64dec2d in vcl::Window::Show(bool, ShowFlags) () from /opt/libreoffice6.2/program/libmergedlo.so
#6  0x00007ffff57a734e in ?? () from /opt/libreoffice6.2/program/libmergedlo.so
#7  0x00007ffff571cf4d in ?? () from /opt/libreoffice6.2/program/libmergedlo.so
#8  0x00007ffff5725e14 in ?? () from /opt/libreoffice6.2/program/libmergedlo.so
#9  0x00007ffff676ce92 in ImplSVMain() () from /opt/libreoffice6.2/program/libmergedlo.so
#10 0x00007ffff57473b5 in soffice_main () from /opt/libreoffice6.2/program/libmergedlo.so
#11 0x000000000040070b in ?? ()
#12 0x00007ffff34192e1 in __libc_start_main (main=0x400700, argc=1, argv=0x7fffffffddb8, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffdda8)
    at ../csu/libc-start.c:291
#13 0x000000000040073f in ?? ()
(gdb)

At the same time SAL_USE_VCLPLUGIN=kde5 works flawlessly.


Steps to Reproduce:
1. Start LibreOffice from command line using following command: 
SAL_USE_VCLPLUGIN=qt5 libreoffice6.2
2. Hanged at splash screen.

Actual Results:
Application hanged at splash screen.

Expected Results:
Successful application startup and main window with recent documents.


Reproducible: Always


User Profile Reset: Yes


OpenGL enabled: Yes

Additional Info:
Using kde5 VCL plugin I've got following:

Version: 6.2.6.1
Build ID: 1f09ad467b449704e317fb11998b9a2ad7184670
CPU threads: 4; OS: Linux 4.9; UI render: default; VCL: kde5; 
Locale: en-US (C); UI-Language: en-US
Calc: threaded